- for [ticket:2651], leaving CheckConstraint alone, preferring to keep

- for [ticket:2651], leaving CheckConstraint alone, preferring to keep
backwards compatibility.  A note about backslashing escapes is added.
Because the Text() construct now supports bind params better, the example
given in the code raises an exception now, so that should cover us.
The exception itself has been enhanced to include the key name of the
bound param.  We're backporting this to 0.8 but 0.8 doesn't have the
text->bind behavior that raises.