Allow c-vol backends to start when some backends fail to load

Allow c-vol backends to start when some backends fail to load

Currently, if a c-vol backend fails to load due to an error in
the module startup process (like a volume driver's __init__ or
dependency import failure), the c-vol process halts completely.

Instead, catch that backend failure, log it, and allow other
backends in enabled_backends to still start up and run.

Note that this is not applicable for failures that allow
drivers to load and remain "uninitialized" -- this is for
failures that occur prior to that point.

Closes-Bug: #1497413

Change-Id: If183b9e3d21777cddf713c2dc88dc80ae7cfe01d