Adapt TypeError Message for String to Integer Conversion.

MRI:
"str" * "3"
=> TypeError: no implicit conversion of String into Integer

mruby:
"str" * "3"
=> TypeError: String can't be coerced into int
Adapt for Float and Integer
Merge pull request #1953 from bovi/conversion-str-int

Adapt TypeError Message for String to (Integer|Float) Conversion