User mappings are essentially anonymous, so messages referring to "user
mapping foo on server bar" are wrong, and inconsistent with other error
messages referring to user mappings. To be consistent with existing use,
use "user mapping for foo on server bar" instead.

I dropped the noise word "user" from the original suggestion to be
consistent with other uses.

Discussion: http://postgr.es/m/56c6f8ab-b2d6-f1fa-deb0-1d18cf67f7b9@2ndQuadrant.com