When the intraline diff fails, the error message simply says that
there was a server error. It could be that the reason for failure
is a timeout, but it's not possible to know this from the error
message.
Add a new error message that is displayed when the intraline diff
fails due to timeout.
Also update the documentation to mention that an error message is
displayed if the intraline diff times out.
Change-Id: I93410d853aadc7ed1f6e4324d0578c4bcbe4e195
4467d2a098