Fix error string format for replication API calls

Fix error string format for replication API calls

enable_replication, disable_replication, and failover_replication all
raise exceptions if the volume they are being called on are not in
valid states. When that is done, the error message is not properly
formatted and looks as such:
"ERROR: Invalid volume: (u'Invalid status to enable replication. valid
states are: %(valid_states)s, current replication-state is:
%(curr_state)s.', {'curr_state': u'enabled', 'valid_states':
['disabled']}) (HTTP 400)"

The reason for this is the msg string is using a ',' instead of a '%'
to evaluate the string. When fixed, the error is displayed correctly:
"ERROR: Invalid volume: Invalid status to enable replication. valid
states are: ['disabled'], current replication-state is: enabled.
(HTTP 400)"

Change-Id: I47dcb0a7688a7d740672a75292e0bf98df1551b2
Closes-Bug: #1504633