Better message for unsupported file extension

Improve the error message for an unsupported output file extension. If
we reach here, we recognize the file extension (currently only .avif),
but the file cannot be the output from avifdec. So the error message
"Unrecognized file extension" is a little confusing.

apps/avifdec.c

diff --git a/apps/avifdec.c b/apps/avifdec.c
index 1af8163..9d25e7e 100644
--- a/apps/avifdec.c
+++ b/apps/avifdec.c
@@ -295,7 +295,7 @@
             returnCode = 1;
         }
     } else {
-        fprintf(stderr, "Unrecognized file extension: %s\n", outputFilename);
+        fprintf(stderr, "Unsupported output file extension: %s\n", outputFilename);
         returnCode = 1;
     }