Test that originalError is set for unique keys

[kevinburkeshyp]

It's set for WLError, but at least one library clobbers the original error
message when creating a WLValidationError. Test that this property exists.

Um, a little uncertain about the best practice here - I only use/care about one library, and I don't expect this to be implemented anywhere else, though I believe it should be, obviously.

Specifically: sails-postgresql currently creates its own error and discards the native Postgres error when a uniqueness check fails. I would like to keep the Postgres error around as originalError, the same way it is for any non-23505 && non-"Key ... already exists" error message. This passes for the sails-postgresql adapter (though many other tests fail; haven't looked into why).

[kevinburkeshyp]

Sorry, this will pass for sails-postgresql once balderdashy/sails-postgresql#180 is merged.

[dmarcelino]

It's not just sails-postgresql but also sails-memory and sails-mysql that are failing. Those will need fixing before we can merge this test.

https://travis-ci.org/balderdashy/waterline-adapter-tests/jobs/75691934#L606

 ------------------------------------------------------------------ 
| adapter          | version | result | failed | total | wl-sequel |
|------------------|---------|--------|--------|-------|-----------|
| sails-postgresql | dfb2229 | failed |      1 |   344 |     0.5.0 |
| sails-memory     | 4281ad7 | failed |      1 |   324 |           |
| sails-disk       | e95628b | passed |      0 |   312 |           |
| sails-mongo      | 021ec02 | passed |      0 |   312 |           |
| sails-mysql      | 1fe7b2c | failed |      1 |   349 |     0.5.0 |
| sails-redis      | ecb7795 | passed |      0 |   312 |           |
 ------------------------------------------------------------------- 

[particlebanana]

@dmarcelino merged balderdashy/sails-postgresql#180 so we just need to look at the others.