Improve handling of IOExceptions in the compiler

[retronym]

• Avoid globalError(ex.getMessage) in compileFiles unless for some reason we're sure the exception message is sufficient for a particular exception. That would be better to deal with by introducing a new custom exception for "message only" failures and throwing that from very particular spots.

• Be more selective about when to report an encoding error in SourceReader.read.

scalac DoesNotExist.scala
error: IO error while decoding DoesNotExist.scala with UTF-8: DoesNotExist.scala (No such file or directory)
Please try specifying another one using the -encoding option
1 error