sanity-check: print installation tips when required module not installed

Signed-off-by: Chase Qi <chase.qi@linaro.org>
diff --git a/validate.py b/validate.py
index 98c5578..1d6d2ed 100755
--- a/validate.py
+++ b/validate.py
@@ -5,14 +5,22 @@
 import subprocess
 import traceback
 import yaml
-import magic
 
 run_pycodestyle = False
 try:
     import pycodestyle
     run_pycodestyle = True
-except ImportError:
-    print("pycodestyle is not available!")
+except ImportError as e:
+    print(e)
+    print("Install pycodestyle: pip3 install pycodestyle")
+    sys.exit(1)
+
+try:
+    import magic
+except ImportError as e:
+    print(e)
+    print("Install python-magic: pip3 install python-magic")
+    sys.exit(1)
 
 
 def print_stderr(message):