srm: Fix return code and logging on "login errors"

A user recently reported that he got the following error:

ERROR MSG:	[FTS] FTS State [Failed] FTS Retries [0] Reason [SOURCE Failed to get source file size: srm-ifce err: Invalid argument, err: [SE][Ls][SRM_AUTHENTICATION_FAILURE] httpg://srm.ndgf.org:8443/srm/managerv2: Authentication failed (server log contains additional information)] Duration [1]

The server log however did not contain additional information, as that
information was only logged at info (which is disabled by default).

This patch refines the exceptions thrown by related code in different
error cases and updates how this is logged and reported to the client.
Thus the client will get more precise error codes and the server log
will contain more relevant information.

Target: trunk
Request: 2.10
Require-notes: yes
Require-book: no