Remove the use of exception in WriteBatch::Handler

Summary:
Remove the use of exception in WriteBatch::Handler.  Now the default
implementations of Put, Merge, and Delete in WriteBatch::Handler are no-op.

Test Plan:
Add three test cases in write_batch_test
./write_batch_test

Reviewers: sdong, igor

Reviewed By: sdong, igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D29835