glsl: Be consistent about '\n', '.', and capitalization in errors/warnings.

The majority of calls to _mesa_glsl_error(), _mesa_glsl_warning(), and
_mesa_glsl_parse_state::check_version() use a message that begins with
a lower case letter and ends without a period.  This patch makes all
messages follow that convention.

Also, error/warning messages shouldn't end in '\n', since
_mesa_glsl_msg() automatically adds '\n' at the end of the message.