`cloud-init analyze` should emit a useful error message if it finds no log lines to process

[Dan Watkins]

When run on the log output from Amazon Linux 2 (see bug 1876323), `analyze` fails to find any log lines to process. Currently, it treats this as a normal situation and emits:

$ cloud-init analyze show -i ~/Downloads/uhJNysgm.txt
-- Boot Record 01 --
The total time elapsed since completing an event is printed after the "@" character.
The time the event takes is printed after the "+" character.

Total Time: 0.00000 seconds

1 boot records analyzed

It would be better if it emitted a clear error message that it didn't find any log lines.

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3677