Use camel case for "DateStyle" in some error messages

Use camel case for "DateStyle" in some error messages

This GUC is written as camel-case in most of the documentation and the
GUC table (but not postgresql.conf.sample), and two error messages
hardcoded it with lower case characters.  Let's use a style more
consistent.

Most of the noise comes from the regression tests, updated to reflect
the GUC name in these error messages.

Author: Peter Smith
Reviewed-by: Peter Eisentraut, Álvaro Herrera
Discussion: https://postgr.es/m/CAHut+Pv-kSN8SkxSdoHano_wPubqcg5789ejhCDZAcLFceBR-w@mail.gmail.com