remove trailing whitespace
add travis badge
tweak erbi error codes
  * remove duplicate 'unknown_object' code
  * add permissions and constraint violation codes
  * fix typo in erbdrv_error_code type def.
re-work postgres error handling
  * use full list of error codes from postgres site
  * map to two atoms, based on 2-letter prefix and full code
  * map to erbi error codes where possible