Gracefully handle build errors in esbuild script

[lpsinger]

We have two plugins which each take the metafile as an argument. However, if the build fails, then the metafile arguemnt will not be set. In this case esbuild will print the original and hopefully helpful error message, but also unhelpful error messages about the metafile being undefined. Suppress the latter error messages by checking first that the metafile is defined.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 6.06%. Comparing base (def8c76) to head (8117a76).
Report is 2 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2805   +/-   ##
=====================================
  Coverage   6.06%   6.06%           
=====================================
  Files        171     171           
  Lines       4334    4334           
  Branches     476     476           
=====================================
  Hits         263     263           
  Misses      4069    4069           
  Partials       2       2           

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