Improve reporting on user errors

[frosch123]

When the user specifies incorrect/insufficient options, the CLI usually prints a long backtrace.
Maybe add --verbose option to print backtraces, and hide them by default.

Examples:

When --api-url is wrong/missing:

• about 60 lines of python backtrace
• aiohttp.client_exceptions.ClientConnectorError: Cannot connect to host api.bananas.openttd.org:443 ssl:default [Name or service not known]

When uploading zero files:
.env/bin/python -m bananas_cli --api-url https://api.bananas.staging.openttd.org upload --version "foobar"

• 25 lines of python backtrace

• File ".../bananas_cli/commands/upload.py", line 31, in upload
  parts = files[0].split("/")[:-1]
  IndexError: tuple index out of range
  

[erenes]

I've fixed this specific issue in the commit mentioned above, but I based it on a branch already containing my PRs #20, #21 and #22.

There will be some merge conflicts if we merge them one by one, so before sending a PR for this, I wanted to check if it would be better to send a new PR that combines all the fixes in one PR, or would you rather merge the other PRs first and then have me rebase off that?

[TrueBrain]

What I commonly do, is make the PR with the others included, and stating that in the Pull Request (something like: include #.., #.., etc). That means for the reviewer it is clear in which order he should review the stuff, and makes sure you don't have to make your life very complicated :D

Also poke to reviewers to get a move on :P