avocado.core.exceptions: Modifications to CmdError for serialization

In order to serialize the CmdError class, we need to
allow passing None command and CmdResult objects to
the class constructor.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

avocado/core/exceptions.py

@@ -106,15 +106,18 @@ class TestWarn(TestBaseException):
 
 class CmdError(Exception):
 
-    def __init__(self, command, result):
+    def __init__(self, command=None, result=None):
         self.command = command
         self.result = result
 
     def __str__(self):
-        if self.result.exit_status is None:
-            msg = "Command '%s' failed and is not responding to signals"
-            msg %= self.command
+        if self.result is not None:
+            if self.result.exit_status is None:
+                msg = "Command '%s' failed and is not responding to signals"
+                msg %= self.command
+            else:
+                msg = "Command '%s' failed (rc=%d)"
+                msg %= (self.command, self.result.exit_status)
+            return msg
         else:
-            msg = "Command '%s' failed (rc=%d)"
-            msg %= (self.command, self.result.exit_status)
-        return msg
+            return "CmdError"