Fixup erroneous `exc` attribute access. (#4818)
Fixup erroneous `exc` attribute access. (#4818)

By inspection, a `ResolveError` has no `exc`, only a `Throw` does. This
looks like it was a simple engine refactor ~typo/copypasta miss in
edd039bf.

Fixes #4812
tdyas/pants,wisechengyi/pants,baroquebobcat/pants,twitter/pants,fkorotkov/pants,foursquare/pants,wisechengyi/pants,baroquebobcat/pants,jsirois/pants,wisechengyi/pants,wisechengyi/pants,benjyw/pants,baroquebobcat/pants,foursquare/pants,foursquare/pants,twitter/pants,UnrememberMe/pants,foursquare/pants,foursquare/pants,UnrememberMe/pants,UnrememberMe/pants,pantsbuild/pants,fkorotkov/pants,wisechengyi/pants,tdyas/pants,twitter/pants,UnrememberMe/pants,twitter/pants,pantsbuild/pants,tdyas/pants,wisechengyi/pants,benjyw/pants,15Dkatz/pants,fkorotkov/pants,UnrememberMe/pants,tdyas/pants,benjyw/pants,15Dkatz/pants,twitter/pants,fkorotkov/pants,UnrememberMe/pants,baroquebobcat/pants,baroquebobcat/pants,pantsbuild/pants,wisechengyi/pants,baroquebobcat/pants,foursquare/pants,fkorotkov/pants,pantsbuild/pants,wisechengyi/pants,baroquebobcat/pants,fkorotkov/pants,UnrememberMe/pants,15Dkatz/pants,foursquare/pants,twitter/pants,fkorotkov/pants,15Dkatz/pants,twitter/pants,15Dkatz/pants,tdyas/pants,wisechengyi/pants,baroquebobcat/pants,benjyw/pants,15Dkatz/pants,benjyw/pants,jsirois/pants,jsirois/pants,tdyas/pants,15Dkatz/pants,UnrememberMe/pants,pantsbuild/pants,foursquare/pants,UnrememberMe/pants,foursquare/pants,tdyas/pants,tdyas/pants,twitter/pants,tdyas/pants,twitter/pants,fkorotkov/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,baroquebobcat/pants,benjyw/pants,15Dkatz/pants