Fix the explanation of HTTPNotFound for cell showing v2 API

This patch adds the explanation to cell showing in v2 API, when
the cell is not found, the exception of HTTPNotFound will return
more infomation.

Change-Id: I25dee08a04ad974af5309999527e0f173d2c2cab
Closes-Bug: #1316152
Merge "Fix the explanation of HTTPNotFound for cell showing v2 API"