Simplify error handling

[stefanhaller]

• 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.

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• Docs (specifically docs/Config.md) have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.03% (target: -2.00%)	✅ 23.55%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (145795c)	50529	42491	84.09%
Head commit (caad553)	50499 (-30)	42483 (-8)	84.13% (+0.03%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#3502)	276	65	23.55%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[jesseduffield]

This is fantastic: the previous situation was indeed terrible and it's going to be much easier to work in the codebase now, thanks!