Sanity check for JImplements

[Thrameos]

Report an easy to make error with a less confusing message.

[codecov[bot]]

Codecov Report

Merging #1082 (c371234) into master (faa41f7) will decrease coverage by 0.01%. The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #1082      +/-   ##
==========================================
- Coverage   88.63%   88.62%   -0.02%     
==========================================
  Files         112      112              
  Lines       10235    10247      +12     
  Branches     4012     4012              
==========================================
+ Hits         9072     9081       +9     
- Misses        703      706       +3     
  Partials      460      460              

Impacted Files	Coverage Δ
jpype/_jproxy.py	97.87% <75.00%> (-1.02%)	:arrow_down:
jpype/_core.py	94.66% <0.00%> (-0.73%)	:arrow_down:

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