Show the revision ID on error message when content is missing on difference page

Currently if the revision exists but its content is not found, it will make
loadText() return false; which in turn will make a call to showMissingRevision()
from showDiff(). However since the revision exists; it will not match the
condition in showMissingRevision() and thus an error message will appear without
any revision ID.

Change-Id: Idd8a5f20a3c082a3b02bd77557e0f4dc4c66e876