added 23503 pgerrcode support and updated the error conversion approach

[amirejaz75]

• [Y ] Do only one thing
• [ Y] Non breaking API changes
• [ Y] Tested

What did this pull request do?

Added 23503 pgerrcode support and updated the error conversion from pg error to gorm error approach

User Case Description

[saeidee]

You may have conflict after merging this https://github.com/go-gorm/postgres/pull/179

[saeidee]

@jinzhu could you please revert this, as it has not properly resolved the conflict with this https://github.com/go-gorm/postgres/pull/179 as well as needs to update gorm version to have the new error type.

[watson-sam]

@saeidee just opened an issue (https://github.com/go-gorm/postgres/issues/187) as hadnt seen yout above comment but this is indeed causing an error

[jinzhu]

@jinzhu could you please revert this, as it has not properly resolved the conflict with this #179 as well as needs to update gorm version to have the new error type.

Can you submit the revert commit and release a new version?

(I haven't something wrong with my development machine ;(

[saeidee]

@codingamir feel free to create a new PR & thanks for contributing.