Convert some Java exceptions to RuntimeError

[khaeru]

Closes #172.
Closes #194.
Closes #236.

These are all, however, workarounds; #296 would be the complete fix.

How to review

Note that the CI checks all pass.

PR checklist

• Tests added.
• Documentation added.
• Release notes updated.

[codecov]

Codecov Report

Merging #362 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #362      +/-   ##
==========================================
+ Coverage   96.54%   96.60%   +0.05%     
==========================================
  Files          45       45              
  Lines        5239     5243       +4     
==========================================
+ Hits         5058     5065       +7     
+ Misses        181      178       -3     

Impacted Files	Coverage Δ
ixmp/backend/jdbc.py	96.39% <100.00%> (+0.55%)	⬆️
ixmp/tests/backend/test_jdbc.py	83.78% <100.00%> (+0.68%)	⬆️

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