pmerge: fix file <-> dir merge override error handling

[radhermit]

Currently when merging a package that tries to replace an existing dir with a file or vice versa something similar to the following traceback is thrown:

Traceback (most recent call last):
  File "/usr/lib/python-exec/python3.7/pmerge", line 4, in <module>
    scripts.run(basename(__file__))
  File "/usr/lib/python3.7/site-packages/pkgcore/scripts/__init__.py", line 34, in run
    sys.exit(tool())
  File "/usr/lib/python3.7/site-packages/snakeoil/cli/tool.py", line 79, in __call__
    ret = self.main()
  File "/usr/lib/python3.7/site-packages/snakeoil/cli/tool.py", line 182, in main
    self.handle_exec_exception(e)
  File "/usr/lib/python3.7/site-packages/snakeoil/cli/tool.py", line 167, in main
    exitstatus = func(self.options, self.out, self.err)
  File "/usr/lib/python3.7/site-packages/pkgcore/scripts/pmerge.py", line 957, in main
    ret = i.finish()
  File "/usr/lib/python3.7/site-packages/snakeoil/currying.py", line 50, in callit
    return func(*(args + moreargs), **morekwargs)
  File "/usr/lib/python3.7/site-packages/snakeoil/dependant_methods.py", line 70, in _ensure_deps
    r = getattr(self, dep).sd_raw_func(self, *a, **kw)
  File "/usr/lib/python3.7/site-packages/pkgcore/operations/domain.py", line 163, in transfer
    merge_phase()
  File "/usr/lib/python3.7/site-packages/pkgcore/merge/engine.py", line 304, in execute_hook
    trigger(self, self.csets)
  File "/usr/lib/python3.7/site-packages/pkgcore/merge/triggers.py", line 131, in __call__
    return self.trigger(engine, *self._get_csets(required_csets, csets))
  File "/usr/lib/python3.7/site-packages/pkgcore/merge/triggers.py", line 432, in trigger
    return op(merging_cset, callback=engine.observer.installing_fs_obj)
  File "/usr/lib/python3.7/site-packages/pkgcore/fs/ops.py", line 287, in merge_contents
    copyfile(x, mkdirs=True)
  File "/usr/lib/python3.7/site-packages/pkgcore/fs/ops.py", line 134, in default_copyfile
    raise CannotOverwrite(obj, existing)
pkgcore.fs.ops.CannotOverwrite: cannot write /usr/bin due to /usr/bin existing
WARNING pkgcore: MutatedPkg(installed pkg: dev-ml/lablgl-1.05-r1::vdb,
               : source repo 'gentoo', overrides=('bdepend', 'depend',
               : 'rdepend', 'pdepend')) -> built ebuild:
               : dev-ml/lablgl-1.06 replacement was underway, but
               : wasn't completed

Instead we should either make a more friendly error message without a traceback or handle it how portage does, i.e. merge the clashing file/dir renamed to something else and show a warning.