NOTE: errstr was renamed to errsym.
Added errsym builtin function. The errsym(errnum | "E_STRING")
builtin, , when given a valid integer errnum that corresponds to a
calc error condition, will return an E_STRING string, AND when given
a valid E_STRING string that is associated with a calc error
condition, will return errnum integer that corresponds to a calc
error condition.
Supplying a non-integer numeric errnum code to error(), errno(),
strerror(), or errsym() will result in an error.
