No compile error marker shown for unmappable characters

Due to a JDK bug (just submitted, internal review ID: 9050071), the compiler says compilation was successful when the only error in the file is an unmappable character. This confuses BlueJ, which expects that the compiler API works as documented. It's going to be a trivial work-around: we need to keep track of whether there was an error, and mark compilation as unsuccessful if so, despite what the compiler API returns. In the mean-time though, symptoms are: if you have a Java file with an unmappable character, compilation will fail but you won't see an error message. Work-around for user is just to trivially edit the file, which saves a valid character, and then proceed from there.