Parser for error and notice fields

[plaflamme]

Postgres errors and "notice" (informational) messages contain a key-value pair of "fields". Currently, we parse the keys, but we don't parse the values.

Each field has a particular semantic, so the value needs to be parsed according to the key.

The possible keys and their meaning is listed here

Their corresponding representation is here

The task is to add parsing for some fields, specifically:

• Severity: to extract error vs. warning, etc
• Code: to extract the "sql state", which is defined here

Once a parser exists, tests should be added:

• MessageDecoderSpec: to test the proper decoding of the values
• SimpleQuerySpec: test that various error codes produce the correct severity and sql state values