[HFX-1070] SCTX-1515: Handling storage exception while unplugging vbd
Slight tidy of Ravi's recent commit, and rephrased log messages.

Based on commit 957e078ad43ed3a30954d331e3263dfd65c69a0c
"[HFX-1070] SCTX-1515: Handling storage exception while unplugging vbd"

Signed-off-by: Thomas Sanders <thomas.sanders@citrix.com>