Improve the error message when creating an empty range partition.

Improve the error message when creating an empty range partition.

The previous message didn't mention the name of the table or the
bounds.  Put the table name in the primary error message and the
bounds in the detail message.

Amit Langote, changed slightly by me.  Suggestions on the exac
phrasing from Tom Lane, David G. Johnston, and Dean Rasheed.

Discussion: http://postgr.es/m/CA+Tgmoae6bpwVa-1BMaVcwvCCeOoJ5B9Q9-RHWo-1gJxfPBZ5Q@mail.gmail.com