Bad error message for pattern constraint

The error message that the YANG parser associates with a pattern (in PatternConstraintImpl) is:

this.errorMessage = errorMessage != null ? errorMessage : String.format(
"String %s is not valid regular expression.", regex);

This is a misleading error message: it is not the regular expression that is invalid, but the given value: that value does not match the regular expression, and so the value is invalid.

This error message should be corrected.