fetch: bb.fatal when trying to checksum non-existing files
    
    If the local fetcher was not able to find the file anywhere but it
    was included in the SRC_URI for checksumming just make it a fatal
    error. Ensure a list of searched locations is included too to match
    the runtime error that would have resulted.
    
    Signed-off-by: Paulo Neves <ptsneves@gmail.com>
    Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>