Return codes for 'patio migrate'

[Jordan Ayers]

It would be helpful for scripting database migrations if 'patio migrate' would return a non-zero value for error conditions. Two error cases that returned a 0 code for me recently:

1. Calling patio migrate with an invalid directory name
2. DB patch application failed due to dereferencing an undefined value in a migration script. (The transaction in the patch was appropriately rolled back.)

Some other cases (such as missing some required arguments) are already returning error codes.

[Doug Martin]

This is fixed in v0.2.15