1. Update get_change_event
   - Return event in a bitmap format in case there is an error
   - Return an extra map indexed by sfp_error in case there is a Mellanox-specific error
   - Indicate whether the error is a blocking error
2. Expose the error status of SFP modules to CLI
   - A platform API calls SDK API to fetch the error status
     - Display the SFP module state: plugged, unplugged and plugged with error.
     - The SFP error state is displayed only if the SFP module state is plugged with error
   - The CLI will call platform API and provide user-friendly output
3. Redirect stderr to /dev/null in order to eliminate errors in case SFP module is not plugged
4. Beautify the code, making all the state/error/descriptions defined in a uniform place

Signed-off-by: Stephen Sun <stephens@nvidia.com>

Signed-off-by: Stephen Sun <stephens@nvidia.com>

Signed-off-by: Stephen Sun <stephens@nvidia.com>

Support handling the error status of SFP modules