Connection.__init__ no longer raises iRODS error code exceptions

[alanking]

In the past, instantiating a Connection object with invalid credentials for authentication would result in an iRODS error code being raised (e.g. PAM_AUTH_PASSWORD_FAILED or CAT_INVALID_USER). Now, any failed authentication causes ValueError to be raised and the "reason" for the failed authentication is lost.

The relevant code change can be found here: 8956644#diff-65db2a2628890f6590ada70f47840fedb056dec02a3e0e61e27a339bea7514d2R80-R89

I'm using the bug label, but this may be intentional and we should write down why this change has occurred if it is expected.

[d-w-moore]

If there were a valid reason for masking the original server-given exception types and codes, it might be to add some consistency to the client interface, as @alanking noted in e-mail discussions today. We should discuss the pros and cons of that here, perhaps, but I'll just note that perhaps we should then:

• wait for the Python3-only future
• use the raise-from syntax to transform those errors into something "more consistent". This sets the __cause__ attribute to preserve the original exception.
• use an Exception type other than ValueError as our error indicator; it's too broad, though probably appropriate in the original usage to identify an invalid scheme here.

[alanking]

I created #545 for considering how to make the errors more uniform.