[IMP] convert PG errors through pg93 diag info

Previously availability of meta-information (failing constraint,
impacted table and column, …) in pg errors (e.g. constraint check
failure) was limited and only available through formatted error
messages, which could be localised (by PG itself) so not did getting
that information require string extraction it was brittle in the face of
localised instances.

Postgres 9.3 adds this meta-information to error diagnostic data
(`PQresultErrorField`), allowing easy programmatic access to it.
Psycopg2 [added a Diagnostic
object](http://initd.org/psycopg/docs/extensions.html#psycopg2.extensions.Diagnostics)
around the time pg9.3 was itself released.

Assuming Odoo now requires pg >= 9.3 we can remove the old string
munging extraction for diagnostic.

* funny story, the handling of 23505 didn't actually work because
  "duplicate key value violates unique constraint" is a literal part
  of the error message, I'd misunderstood "value" as a field name
  because my test model's field was called "value". Makes sense too
  since you can set UNIQUE constraints on multiple fields (and UNIQUE
  indices on expressions)
* for both errors, it should be possible to use the table_name or
  constraint metadata to provide messages about sub-model issues (a
  constraint on an m2o), but that's not currently handled

Fixes #15323

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