Allow commands to return error codes.

bin/spack

@@ -103,7 +103,7 @@ if args.insecure:
 # Try to load the particular command asked for and run it
 command = spack.cmd.get_command(args.command)
 try:
-    command(parser, args)
+    return_val = command(parser, args)
 except SpackError, e:
     if spack.debug:
         # In debug mode, raise with a full stack trace.
@@ -116,3 +116,11 @@ except SpackError, e:
 except KeyboardInterrupt:
     sys.stderr.write('\n')
     tty.die("Keyboard interrupt.")
+
+# Allow commands to return values if they want to exit with some ohter code.
+if return_val is None:
+    sys.exit(0)
+elif isinstance(return_val, int):
+    sys.exit(return_val)
+else:
+    tty.die("Bad return value from command %s: %s" % (args.command, return_val))