allow user to specify max size of error messages

Errors can have messages that are very large. When they are raised,
redis space is rapidly depleted.

An example of this problem is an INSERT or UPDATE in ActiveRecord
with a very large blob of data. When ActiveRecord fails, it puts
the entire SQL statement into the error message.
Merge pull request #1843 from covermymeds/truncate-huge-error-messages

Allow user-specified maximum error message size