In order to support PostgreSQL <= 9.5, the b'S' field of an error/notice
message was parsed. However, this field can be localized and thus parsing
can fail for instances that use a non-english locale. In version > 9.5,
the b'V' field, that is guaranteed to be in english, was added. However,
even for these versions parsing would fail as the b'S' field was also
parsed. This patch prefers b'V' over b'S' if it exists and uses a default
severity in case b'V' is not present and b'S' could not be parsed.
Fixes#734