fix ZipException

```

net.lingala.zip4j.exception.ZipException: could not find file header for file: iii/bbb/Copy of Foo.ceylon
	at net.lingala.zip4j.core.ZipFile.removeFile(ZipFile.java:791)
	at com.redhat.ceylon.eclipse.core.builder.CeylonBuilder.cleanRemovedFilesFromOutputs(CeylonBuilder.java:3232)
```
This happens when the user deletes a Ceylon source file that has never compiled, when the IDE tries to remove the source file from the src archive (but the source has never been added to the archive).