Update HaveErrorsMatcher.

Current spec status: 248 examples, 12 failures in 0.07776 seconds.
Remove support for local symlinks to nested directories.
Remove custom_double mock method.

Current spec status:
1.9.3: 196 examples, 0 failures in 0.06622 seconds.
2.0.0: 211 examples, 0 failures in 0.07339 seconds.
2.1.0: 234 examples, 0 failures in 0.07601 seconds.