S3FS: check for AWS_* environ variables and fail early if missing.

This closes issue #41.

fs/s3fs.py

@@ -9,6 +9,7 @@ interface for objects stored in Amazon Simple Storage Service (S3).
 
 """
 
+import os
 import time
 import datetime
 import tempfile
@@ -88,6 +89,12 @@ class S3FS(FS):
             prefix = prefix[1:]
         if not prefix.endswith(separator) and prefix != "":
             prefix = prefix + separator
+        if aws_access_key is None:
+            if "AWS_ACCESS_KEY_ID" not in os.environ:
+                raise CreateFailedError("AWS_ACCESS_KEY_ID not set")
+        if aws_secret_key is None:
+            if "AWS_SECRET_ACCESS_KEY" not in os.environ:
+                raise CreateFailedError("AWS_SECRET_ACCESS_KEY not set")
         self._prefix = prefix
         self._tlocal = thread_local()
         super(S3FS, self).__init__(thread_synchronize=thread_synchronize)