Improve error message in generate_query_config.pl

Add usage information to the ARGV-incorrect-length error
message in generate_query_config.pl. A plain usage message
looks a bit incongruous when raised as an error, but the
error message alone is unhelpful.

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/scripts/generate_query_config.pl b/scripts/generate_query_config.pl
index b565024..7855c7c 100755
--- a/scripts/generate_query_config.pl
+++ b/scripts/generate_query_config.pl
@@ -37,7 +37,7 @@
 my ($config_file, $query_config_format_file, $query_config_file);
 
 if( @ARGV ) {
-    die "Invalid number of arguments" if scalar @ARGV != 3;
+    die "Invalid number of arguments - usage: $0 [CONFIG_FILE TEMPLATE_FILE OUTPUT_FILE]" if scalar @ARGV != 3;
     ($config_file, $query_config_format_file, $query_config_file) = @ARGV;
 
     -f $config_file or die "No such file: $config_file";