check --ssl-cert and --ssl-key to be files

httpie/utils.py

@@ -1,4 +1,5 @@
 from __future__ import division
+import argparse
 
 
 def humanize_bytes(n, precision=2):
@@ -44,3 +45,12 @@ def humanize_bytes(n, precision=2):
             break
 
     return '%.*f %s' % (precision, n / factor, suffix)
+
+
+def existing_file(filename):
+    try:
+        open(filename, 'rb')
+    except IOError as ex:
+        raise argparse.ArgumentTypeError(
+            '%s: %s' % (filename, ex.args[1]))
+    return filename