More robust error message on method compilation failure

[wsmoses]

If the method compiling a generated function fails, it will throw a vanilla unhelpful assertion.

By adding more information to the assertion error itself, its easier to figure out what went wrong and why

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.00%. Comparing base (47da204) to head (c954cde).

Files with missing lines	Patch %	Lines
src/jlgen.jl	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #738      +/-   ##
==========================================
- Coverage   75.07%   75.00%   -0.07%     
==========================================
  Files          24       24              
  Lines        3639     3641       +2     
==========================================
- Hits         2732     2731       -1     
- Misses        907      910       +3     

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[maleadt]

You can just pass an additional message to @assert