handle notimplementederror exception for the plugins

[lguohan]

in sfputil, gracefully catch notimplementederror exception throw by plugins and print out error instead of throw traceback stack.

admin@str-msn2700-20:/usr/share/sonic/device/x86_64-mlnx_msn2700-r0/plugins$ sudo sfputil show lpmode
Traceback (most recent call last):
  File "/usr/bin/sfputil", line 9, in <module>
    load_entry_point('sonic-utilities==1.1', 'console_scripts', 'sfputil')()
  File "/usr/lib/python2.7/dist-packages/click/core.py", line 716, in __call__
    return self.main(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/click/core.py", line 696, in main
    rv = self.invoke(ctx)
  File "/usr/lib/python2.7/dist-packages/click/core.py", line 1060, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/lib/python2.7/dist-packages/click/core.py", line 1060, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/lib/python2.7/dist-packages/click/core.py", line 889, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/lib/python2.7/dist-packages/click/core.py", line 534, in invoke
    return callback(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/sfputil/main.py", line 514, in lpmode
    lpmode = platform_sfputil.get_low_power_mode(physical_port)
  File "/usr/share/sonic/device/x86_64-mlnx_msn2700-r0/plugins/sfputil.py", line 54, in get_low_power_mode
    raise NotImplementedError
NotImplementedError