AUTO-407 fix: Giving 400 validation error instead of 500

Since the errors are caused by croniter throwing exceptions other than ValueError, I am catching
Exception to handle these errors.

Also, this does not solve seconds problem since croniter considers 6th member for seconds.
Merge branch 'master' into cron_fix

Conflicts:
	otter/json_schema/group_schemas.py
Not allowing seconds in cron

Also added tests for valid and invalid cron entries. However, following 2 issues still remain due to bug
in croniter:
'* * * * 7' works (last element is between 0 to 6)
'* * 0 * *' works (day of the month should be 1-31)
Description about catching Exception instance

while validation cron entries. Also raised issue with croniter about the same and referenced it here
Adding description about catching Exception when validaing iso8601 datetime
Adding croniter issues not working in comment
raising ValueError on catching blanket exception
works with empty 'at'
Merge branch 'master' into cron_fix
Tests checking 407 now running
moved 6 star cron test to negative

since seconds is allowed in cron now
Merge pull request #257 from rackerlabs/cron_fix

AUTO-407 fix: Giving 400 validation error instead of 500