Improved error logging in nova-network for allocate_fixed_ip()

When a Nova deployment runs out of fixed IP addresses, allocating
another IP ends with stack trace without clear explanation.
To prevent such behavior this patch introduces few try-except blocks
that writes appropriate error message to the log file and return
better error code via API instead of stack trace.

Change-Id: I8111ae883719ad693113e1d3bfedc8685427c55f
Closes-Bug: #1218572
Merge "Improved error logging in nova-network for allocate_fixed_ip()"