Corrected an error message formatting problem.

MessageFormat.format() expands numeric arguments differently to strings.
For example, given the call MessageFormat.format("{0}", x), where "x" is
an integer of value 2147483647 results in a string containing
"2,147,483,647". If, however, the call is MessageFormat("{0}",
x.toString), then the result is a string containing "2147483647".
However, since Scala Ints are different to Java Integers, there are
further problems. To get around this, all calls to
MessageFormat.format() convert value arguments explicitly to strings.