PARQUET-331: Surface subprocess stderr in merge script

This makes it a little easier to understand why the merge script failed

Author: Alex Levenson <alexlevenson@twitter.com>

Closes #240 from isnotinvain/alexlevenson/merge-script-error-handling and squashes the following commits:

7c38c01 [Alex Levenson] Surface subprocess stderr in merge script

dev/merge_parquet_pr.py

@@ -79,11 +79,19 @@ def fail(msg):
 
 
 def run_cmd(cmd):
-    if isinstance(cmd, list):
-        return subprocess.check_output(cmd)
-    else:
-        return subprocess.check_output(cmd.split(" "))
-
+    try:       
+        if isinstance(cmd, list):
+            return subprocess.check_output(cmd, stderr=subprocess.STDOUT)
+        else:
+            return subprocess.check_output(cmd.split(" "), stderr = subprocess.STDOUT)          
+    except subprocess.CalledProcessError as e:
+        # this avoids hiding the stdout / stderr of failed processes
+        print 'Command failed: %s' % cmd
+        print 'With output:'
+        print '--------------'
+        print e.output
+        print '--------------'
+        raise e
 
 def continue_maybe(prompt):
     result = raw_input("\n%s (y/n): " % prompt)