Improve style guideline compliance of assorted error-report messages.

Improve style guideline compliance of assorted error-report messages.

Per the project style guide, details and hints should have leading
capitalization and end with a period.  On the other hand, errcontext should
not be capitalized and should not end with a period.  To support well
formatted error contexts in dblink, extend dblink_res_error() to take a
format+arguments rather than a hardcoded string.

Daniel Gustafsson

Discussion: https://postgr.es/m/B3C002C8-21A0-4F53-A06E-8CAB29FCF295@yesql.se