Provide better messages when redfish provides

This provides a more informative behavior in the clients.

Change-Id: I7cecc39c3928aac6ee2d9b86ed53159dcdf3dde5

pyghmi/exceptions.py

@@ -29,6 +29,10 @@ class IpmiException(PyghmiException):
         self.ipmicode = code
 
 
+class RedfishError(PyghmiException):
+    pass
+
+
 class UnrecognizedCertificate(Exception):
     def __init__(self, text='', certdata=None):
         super(UnrecognizedCertificate, self).__init__(text)

pyghmi/redfish/command.py

@@ -312,7 +312,15 @@ class Command(object):
             if 'If-Match' in wc.stdheaders:
                 del wc.stdheaders['If-Match']
         if res[1] < 200 or res[1] >= 300:
-            raise exc.PyghmiException(res[0])
+            try:
+                info = json.loads(res[0])
+                errmsg = [
+                    x['Message'] for x in info.get('error', {}).get(
+                        '@Message.ExtendedInfo', {})]
+                errmsg = ','.join(errmsg)
+                raise exc.RedfishError(errmsg)
+            except (ValueError, KeyError):
+                raise exc.PyghmiException(res[0])
         if payload is None and method is None:
             self._urlcache[url] = {'contents': res[0],
                                    'vintage': os.times()[4]}