#106 No error when output files could not be copied

[Michiel Hendriks]

When the resulting files could not be copied to the
output location only a warning (file not found) is
issued. I think this should be an error rather than a
warning.

For example if the file was in use (because the old
program is still running) because the program is still
running the build will be succesfull. But running the
compose* program will start the previous build.