feat: better error message printing when signature is missing

[baszalmstra]

Adds impl fmt::Display in the ABI autogen to get more correct descriptive error messages

[codecov]

Codecov Report

Merging #80 into master will decrease coverage by 0.14%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master     #80      +/-   ##
=========================================
- Coverage   74.75%   74.6%   -0.15%     
=========================================
  Files         117     117              
  Lines        8868    8885      +17     
=========================================
  Hits         6629    6629              
- Misses       2239    2256      +17

Impacted Files	Coverage Δ
crates/mun_abi/src/autogen_impl.rs	94.28% <0%> (-4.36%)	⬇️

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