Fix exception message in BlobStoreUtil's getSnapshotIndex

[ajothomas]

Bug:
BlobStoreUtil's getSnapshotIndex method currently gets the SnapshotIndex blob using BlobStoreManager#get and deserializes it using SnapshotIndexSerde. We handle the exceptions encountered in the get step and deserialization step using the chained handle step.
The error log ,however, tells thats the exception was due to deserialization. This is incorrect as the handle will get exceptions from any of the previous steps i.e get and not just the deserialization step.

Changes:

• Change handle message to reflect the correct exception message

Tests:

• ./gradlew build
• Added unit tests to assert if appropriate exception is thrown

API Changes:
None

Upgrade Instructions:
None

Usage Instructions:
None

[xinyuiscool]

LGTM! Thanks for the update.