insufficient error handling

[Ben Elliston]

I'm not sure how to go about fixing this without spending lots of time
investigating.  If someone can point me in the right direction, I'm
willing to try and fix this bug myself.

When parsing a .cpu file, an invalid semantic body like:

     (foo (sub op1 op2))

where `foo' is not a known pmacro and is obviously not a valid rtx, I
get:

     ERROR: assertion failure: (rtx? rtx)

It would be nice if the error message told me that `foo' is not a
valid rtx.

Ben