Raise HTTPInternalServerError when boot_from_volume with cinder down

This patch changes exception type.
Boot_from_volume api raises BlockDeviceMappingIsInvalid exception
when cinder-api is down.
Also it returns response code 400.
It is not exact. Because request is valid.
Exception was changed to raise HTTPInternalServerError and
return a 500 error.

Closes-Bug: #1239952

Change-Id: Iec8839d46e6186fe544b28b0aafc8aa6faad53b7
Merge "Raise HTTPInternalServerError when boot_from_volume with cinder down"