ada: Adjust number of errors when removing warning in dead code

When a warning about a runtime exception is emitted for a code in
generic instance, we add continuation warnings "in instantiation ..."
and only the original message increase the total number of errors.

When removing these messages, e.g. after detecting that the code inside
generic instance is dead, we must decrease the total number of errors,
as otherwise the compiler exit status might stop gnatmake or gprbuild.

gcc/ada/

	* errout.adb (To_Be_Removed): Decrease total number of errors when
	removing a warning that has been escalated into error.
	* erroutc.adb (dmsg): Print Warn_Runtime_Raise flag.