Fix #1001394 Ignore source classpath entries that don't exist physically in the workspace.
m2e routinely adds optional CPEs for src/main/java, src/test/java even when
the directories are missing. SBT builder is not aware that they are optional
and aborts compilation.
Simplified ensureFolderExists function according to Iulian's suggestion.
Merge pull request #262 from rkrzewski/master

Ignore non-existent source classpath entries