Turn AnnifException into an abstract base class and fix message prefix behavior

[osma]

I realized that AnnifException isn't intended to be instantiated directly but doing so was still possible. This PR changes into an abstract base class, which cannot be instantiated.

While performing the change, I noticed that the prefix mechanism used to generate error messages was broken. This PR fixes it as well.

Example error before the changes:

Error: Project 'noanalyzer': vocab setting is missing

Example error after the changes:

Error: Misconfigured project 'noanalyzer': vocab setting is missing

[codecov]

Codecov Report

Merging #320 into master will increase coverage by 0.06%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master     #320      +/-   ##
==========================================
+ Coverage   99.38%   99.45%   +0.06%     
==========================================
  Files          55       55              
  Lines        2920     2917       -3     
==========================================
- Hits         2902     2901       -1     
+ Misses         18       16       -2

Impacted Files	Coverage Δ
annif/exception.py	95.23% <80%> (+7.73%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b660eed...475bba0. Read the comment docs.

[osma]

The abstractness isn't actually working though. It's still possible to instantiate AnnifException when defined this way. Apparently inheriting from ClickException somehow cancels the abstractness of the class...

[osma]

Something weird is happening here, I posted a question on StackOverflow because I can't understand it.

[juhoinkinen]

This SO discussion seems to be related.

[osma]

Thanks @juhoinkinen , I wonder why I didn't find that discussion myself.

I answered my own SO question and will update this PR according to the workarounds provided in the earlier SO posts.

[osma]

Almost forgot to add the tests I wrote.