Replace deprecated `error_chain` crate with `anyhow` (#1405)

The `error_chain` crate is now deprecated for a long time and `anyhow`
has proven to be a popular replacement for applications.

This also improves the current error messages for panics.

```
PAGER='"less' git show
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: \
Error(Msg("Could not parse pager command."), State { next_error: Some(ParseError), \
backtrace: InternalBacktrace })', src/main.rs:136:88
```

```
PAGER='"less' git show
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Could not parse pager command.

Caused by:
    missing closing quote', src/main.rs:125:88
```

1 files changed, 1 insertions, 7 deletions

diff --git a/src/main.rs b/src/main.rs
index 72586939..caf94cac 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -49,13 +49,7 @@ where
 }
 
 pub mod errors {
-    error_chain::error_chain! {
-        foreign_links {
-            Io(::std::io::Error);
-            SyntectError(::syntect::LoadingError);
-            ParseIntError(::std::num::ParseIntError);
-        }
-    }
+    pub use anyhow::{anyhow, Context, Error, Result};
 }
 
 #[cfg(not(tarpaulin_include))]