Corrected log message for invalid volume names

The old error message was confusing so corrected the
same to clearly mention if the volume name is not valid.

This could happen in two ways -
1. If the value for fs.glusterfs.volumes contains some invalid volume
names (e.g. some random string which is not a valid volume name)
and so there would be no entry available in the form of the property
fs.glusterfs.volume.fuse.<Volume Name>
2. If the value for fs.glusterfs.volumes contains valid strings which
are valid volume names but corresponding entry is not available in the
form of the property fs.glusterfs.volume.fuse.<Volume Name>

Signed-off-by: Shubhendu Tripathi <shtripat@redhat.com>
Bug-URL: https://bugzilla.redhat.com/show_bug.cgi?id=1101937
Merge pull request #114 from shtripat/fix-1101937

Fix 1101937