exception: Update "TargetNotRespondingError" syntax.

Update the syntax of the TargetNotRespondingError to conform with the
other exceptions of expecting a full message to be displayed rather than just
a target name.

devlib/exception.py

@@ -25,9 +25,7 @@ class TargetError(DevlibError):
 
 class TargetNotRespondingError(DevlibError):
     """The target is unresponsive."""
-
-    def __init__(self, target):
-        super(TargetNotRespondingError, self).__init__('Target {} is not responding.'.format(target))
+    pass
 
 
 class HostError(DevlibError):

devlib/target.py

@@ -455,7 +455,7 @@ class Target(object):
             return 1
         except (TimeoutError, subprocess.CalledProcessError, TargetError):
             if explode:
-                raise TargetNotRespondingError(self.conn.name)
+                raise TargetNotRespondingError('Target {} is not responding'.format(self.conn.name))
             return 0
 
     # process management