Error if --solcoverjs passed but file is nonexistent

[area]

It's been a while 😅 - hope things are good on your side.

While reworking my test suite, I lost a lot of time to this peculiarity and a hard-to-spot typo on my side, so thought I'd see if there was an appetite to saving other people the bother down the line. I realise this is a breaking change though; would be happy to turn it in to a warning rather than an outright failure, if that's a concern.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.63%. Comparing base (04d178e) to head (b9e4677).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #889   +/-   ##
=======================================
  Coverage   96.62%   96.63%           
=======================================
  Files          19       19           
  Lines        1066     1069    +3     
=======================================
+ Hits         1030     1033    +3     
  Misses         36       36           

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[cgewecke]

Hi! 👋

Thanks for this, definitely how it should be :)