Simplify error handling (#3502)

- **PR Description**

Simplify and canonicalize error handling across the code base.

Previously it was important to make sure that errors don't bubble up
into gocui, because it would panic there; so we had to show errors in
error panels in the right places (in controller code, usually). This is
error-prone because it's easy to forget (see #3490 for an example);
also, it's not always obvious where in the call chain the error panel
needs to be shown. Most of the time it's in controller code, but we had
plenty of calls to `Error()` in helpers, and I remember that I found
this very confusing when I was new to the code base.

Change this so that you can simply return errors everywhere. The
functions to show an error message manually have been removed for
clarity.

I tried to structure the commits so that you can review them one by one.

Closes #3491.

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