Fix the explanations of HTTPNotFound for keypair's API

Gets the "explanation" from HTTPNotFound exception and returns
correct message. the commit Ia784e09cda986a421e83c2820dfd0a2267867cdd
has fixed this kind of bug already, but this bug is remained.

Closes-Bug: #1307338
Change-Id: I6811eecba86938e4253f437b7677cb7e4468fd67
Merge "Fix the explanations of HTTPNotFound for keypair's API"