Improve InvalidCircuitSize error message

[moCello]

Resolves #792

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (1f655c2) 85.65% compared to head (a7270b0) 85.63%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #793      +/-   ##
==========================================
- Coverage   85.65%   85.63%   -0.02%     
==========================================
  Files          57       57              
  Lines        4119     4128       +9     
==========================================
+ Hits         3528     3535       +7     
- Misses        591      593       +2     

Files	Coverage Δ
src/composer.rs	90.93% <100.00%> (+0.36%)	⬆️
src/error.rs	8.33% <33.33%> (+8.33%)	⬆️
tests/error_size.rs	72.00% <50.00%> (ø)

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[xevisalle]

LGTM