CRM-20858: Remove Try/Catch Blocks that Ignored Caught Exceptions

CRM-20858: Remove Try/Catch Blocks that Ignored Caught Exceptions

The try catch blocks were apparently added because not all fields passed were
custom fields, generating exceptions when the API call was made to obtain its
option group. This happened because actually all fields selected for merge
were being inserted into the $submittedCustomFields array, instead of just
custom fields. Plus, obtaining the field's ID on a field that was not custom
rendered strange strings to be used on search of the field, as this ID is
obtained by getting a substring from the field's key, that should have the
format 'move_custom_<fieldid>' (fields that are not custom don't have that
format).