signalling errors

[Masinter.pa@Xerox.COM]

I think that changing "is an error" to "signals an error" is premature
until the error-handler is adopted and we can actually standardize
*which error* is signalled.

Before there is an error standard, neither phrase has particularly
well-defined semantics, and choosing between one and the other is
(nearly) moot.

After there is an error standard, there will be much more to decide for
each one. Further, the "hardship" question is much more restrictive --
implementations may not find it hard to signal some error, but there may
be many more difficulties in getting implementations to agree on the
error type.