Give a useful error message if wrong number of commandline args is given

Output before when run with no arguments:

    Traceback (most recent call last):
      File "../ybd/ybd.py", line 30, in <module>
        target = os.path.splitext(os.path.basename(sys.argv[1]))[0]
    IndexError: list index out of range

Output after:

    Usage: ybd.py DEFINITION_FILE ARCHITECTURE

1 files changed, 5 insertions, 0 deletions

diff --git a/ybd.py b/ybd.py
index deae5b7..148b233 100755
--- a/ybd.py
+++ b/ybd.py
@@ -27,6 +27,11 @@ from assembly import assemble, deploy
 import sandbox
 
 
+if len(sys.argv) != 3:
+    sys.stderr.write("Usage: %s DEFINITION_FILE ARCHITECTURE\n" % sys.argv[0])
+    sys.exit(1)
+
+
 print
 target = sys.argv[1]
 arch = sys.argv[2]