[Ada] Eliminate false positives on "no entities ..." message

[Arnaud Charlet <charlet at adacore dot com>]

This change avoids issuing the "no entities .." message for a with'ed
package if serious errors have been found. This eliminates some annoying
false positives, as shown by the following example, compiled with -gnatwa

     1. with System; use System;
     2. package WarnNoEnt is
     3.    X : Integer := "ABC";
                          |
        >>> expected type "Standard.Integer"
        >>> found a string type

     4.    Y : Integer := 0;
     5.     for Y'Address use X'Address;
     6. end WarnNoEnt;

Before this change, line 1 issued a "no entities" warning

Tested on x86_64-pc-linux-gnu, committed on trunk

2014-01-22  Robert Dewar  <dewar@adacore.com>

	* sem_warn.adb (Check_Use_Clause): Don't give no entities used
	msg if errors found.
	(Check_One_Unit): Same change.

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