feat: use `anyhow` for more structured error messages and better error reporting flexibility

This significantly changes the structure of potential output of
top-level errors.  Before, errors that have a `source` were printed on a
single line, with the `source` omitted. Now, an error with a `source`
will be printed like:

```
Error: <`Display` impl of error>

Caused by:
    <`Display` impl of `source` error>
```

If there are multiple `source` errors in a chain, then it will be of the
form:

```
Error: <`Display` impl of error>

Caused by:
    0: <`Display` impl of first `source` error>
    1: <`Display` impl of `source` of `source` error>
    # etc.
```

Now, in practice this never happened before, since the only error
type that ever could return from `<binary>::run` was `std::io::Error`,
which has no `source`. However, `source`s can start being added if, say,
future work involved using `anyhow::Context::context` or custom errors
that implement `std::Error::source` are added to the codebase. I believe
this is a good choice going forward, but I definitely want to foster
discussion and acceptance for it in the open first!

4 files changed, 14 insertions, 4 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 2a61396..ca22315 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -17,6 +17,11 @@ dependencies = [
 ]
 
 [[package]]
+name = "anyhow"
+version = "1.0.31"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
 name = "atty"
 version = "0.2.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -58,6 +63,7 @@ name = "hexyl"
 version = "0.8.0"
 dependencies = [
  "ansi_term 0.12.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "anyhow 1.0.31 (registry+https://github.com/rust-lang/crates.io-index)",
  "atty 0.2.14 (registry+https://github.com/rust-lang/crates.io-index)",
  "clap 2.33.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "libc 0.2.71 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -118,6 +124,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 [metadata]
 "checksum ansi_term 0.11.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ee49baf6cb617b853aa8d93bf420db2383fab46d314482ca2803b40d5fde979b"
 "checksum ansi_term 0.12.1 (registry+https://github.com/rust-lang/crates.io-index)" = "d52a9bb7ec0cf484c551830a7ce27bd20d67eac647e1befb56b0be4ee39a55d2"
+"checksum anyhow 1.0.31 (registry+https://github.com/rust-lang/crates.io-index)" = "85bb70cc08ec97ca5450e6eba421deeea5f172c0fc61f78b5357b2a8e8be195f"
 "checksum atty 0.2.14 (registry+https://github.com/rust-lang/crates.io-index)" = "d9b39be18770d11421cdb1b9947a45dd3f37e93092cbf377614828a319d5fee8"
 "checksum bitflags 1.2.1 (registry+https://github.com/rust-lang/crates.io-index)" = "cf1de2fe8c75bc145a2f577add951f8134889b4795d47466a54a5c846d691693"
 "checksum clap 2.33.1 (registry+https://github.com/rust-lang/crates.io-index)" = "bdfa80d47f954d53a35a64987ca1422f495b8d6483c0fe9f7117b36c2a792129"
diff --git a/Cargo.toml b/Cargo.toml
index 5c42e2c..214480d 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -12,6 +12,7 @@ edition = "2018"
 
 [dependencies]
 ansi_term = "0.12"
+anyhow = "1.0.31"
 atty = "0.2"
 libc = "0.2"
 
diff --git a/src/bin/hexyl.rs b/src/bin/hexyl.rs
index eb43151..7025a71 100644
--- a/src/bin/hexyl.rs
+++ b/src/bin/hexyl.rs
@@ -9,9 +9,11 @@ use clap::{App, AppSettings, Arg};
 
 use atty::Stream;
 
+use anyhow::{anyhow, Error as AnyhowError};
+
 use hexyl::{BorderStyle, Input, Printer};
 
-fn run() -> Result<(), Box<dyn std::error::Error>> {
+fn run() -> Result<(), AnyhowError> {
     let app = App::new(crate_name!())
         .setting(AppSettings::ColorAuto)
         .setting(AppSettings::ColoredHelp)
@@ -165,7 +167,7 @@ fn run() -> Result<(), Box<dyn std::error::Error>> {
 
     let mut printer = Printer::new(&mut stdout_lock, show_color, border_style, squeeze);
     printer.display_offset(display_offset);
-    printer.print_all(&mut reader)?;
+    printer.print_all(&mut reader).map_err(|e| anyhow!(e))?;
 
     Ok(())
 }
@@ -189,7 +191,7 @@ fn main() {
                 _ => (),
             }
         } else {
-            eprintln!("Error: {}", err);
+            eprintln!("Error: {:?}", err);
         }
         std::process::exit(1);
     }
diff --git a/src/lib.rs b/src/lib.rs
index fba1eb3..2bdbdc3 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -408,7 +408,7 @@ impl<'a, Writer: Write> Printer<'a, Writer> {
     pub fn print_all<Reader: Read>(
         &mut self,
         mut reader: Reader,
-    ) -> Result<(), Box<dyn std::error::Error>> {
+    ) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
         let mut buffer = [0; BUFFER_SIZE];
         'mainloop: loop {
             let size = reader.read(&mut buffer)?;