Update and correct documentation snippets

Partially implements: blueprint example-snippet-generator
Change-Id: I26f63bd9ef4a1ec5559d41bda7c476ba829b1729
Fix exception handling in get_replication_snapshot

Unexpected errors that occur while generating the replication snapshot
(for example, TemplatesNotFound) are not logged. This can lead to
accidental hiding of the root cause of a problem.

All exceptions are now written to the log and then re-thrown.

Change-Id: I48de0e099494b204a8b109caedb4b86a616024f1
Closes-Bug: #1386875
Increase instances.task_description column size

The instances.task_description column size is too small to use.
This fix increases it to 255.

The migration is totally safe and the downgrade operation
is not needed.

Change-Id: I658e0dea3d304409085162ec4a2b79f8473a71c4
Closes-Bug: #1334795
Merge "Update and correct documentation snippets"