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

This bug was originally filed in Launchpad as [LP: #1876795](https://bugs.launchpad.net/cloud-init/+bug/1876795)

Launchpad details

affected_projects = []

assignee = None

assignee_name = None

date_closed = None

date_created = 2020-05-04T19:12:58.398935+00:00

date_fix_committed = None

date_fix_released = None

id = 1876795

importance = medium

is_complete = False

lp_url = https://bugs.launchpad.net/cloud-init/+bug/1876795

milestone = None

owner = oddbloke

owner_name = Dan Watkins

private = False

status = triaged

submitter = oddbloke

submitter_name = Dan Watkins

tags = []

duplicates = []

_Launchpad user **Dan Watkins(oddbloke)** wrote on 2020-05-04T19:12:58.398935+00:00_

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.