LET errors

[RWK at MIT-MC (Robert W. Kerns)]

(LET ((FOO (BAR) (BAR))) FOO)
prints far too much in the error message --- everything after the LET!
Clearly it would be much more helpful to print just the part that was
found to be bad.

Also, if you look at the EVALFRAME just below the ERRFRAME, the frame
of the ERROR, you find this random #7171 pointer where you'd expect to
see the rest of the arguments to ERROR.