Print the missing provider information to stderr.

src/dispatch_common.c

@@ -492,10 +492,11 @@ epoxy_print_failure_reasons(const char *name,
 {
     int i;
 
-    printf("No provider of %s found.  Requires one of:\n", name);
+    fprintf(stderr, "No provider of %s found.  "
+            "Requires one of:\n", name);
 
     for (i = 0; providers[i] != 0; i++)
-        puts(provider_names[providers[i]]);
+        fputs(stderr, provider_names[providers[i]]);
 }
 
 PUBLIC void