Improve diagnostics for negative repeated text count in SpEL

[sbrannen]

Due to the changes in #31341, if the repeat count in a SpEL expression (using the repeat operator *) is negative, we throw a SpelEvaluationException with the MAX_REPEATED_TEXT_SIZE_EXCEEDED message which is incorrect and misleading.

Prior to #31341, a negative repeat count resulted in an IllegalArgumentException being thrown by String#repeat(), which was acceptable in terms of diagnostics, but that did not make it immediately clear to the user what the underlying cause was.

In light of the above, we should improve diagnostics for a negative repeated text count in SpEL expressions.