#834 Improve error message

[Walter]

Numeric Digits 4
x=1.23456e126
say format(x,4,4,2)

i:\mark>rexx 126
3 - say format(x,4,4,2)
Error 93 running I:\mark\126.rex line 3: Incorrect call to method.
Error 93.941: Exponent of "1.235" is too large for 2 spaces.

Much better:

I:\mark>regina 126
3 +++ say format(x,4,4,2)
Error 40 running "I:\mark\126.rex", line 3: Incorrect call to routine
Error 40.38: FORMAT argument 4 is not large enough to format "1.23456E126"