Adding GDSCodes

[pomgui]

This PR adds:

1. gdscode and gdsparams to database errors.
2. GDSCode.* constants

Background

When database errors happen, sometimes it's necessary to make decisions based on the error type. Until now the only solution was parsing the error message in order to get the error type and additional data (possibly the constraint name, field, table, etc.)

Error message:

{
  message: 'Error: violation of PRIMARY or UNIQUE KEY constraint "INTEG_164" on table "SPEC_TEST", Problematic key value is ("TEST_ID" = 1)'
}

With the GDS codes in the error object, this is much easier task.

Error object:

{
  gdscode: 335544665,
  gdsparams: ["INTEG_164","SPEC_TEST"],
  message: "violation of PRIMARY or UNIQUE KEY constraint \"INTEG_164\" on table \"SPEC_TEST\", Problematic key value is (\"TEST_ID\" = 1)"
}
// Associated constant: GDSCode.UNIQUE_KEY_VIOLATION = 335544665

The reason for these constants are not exported within the main index.js is that they require memory space which only will be used if they're really needed. That's also the reason to return a numeric gdscode instead of a mnemonic string (that could be trivial to get inside the index.js).