Better message for machines validation on bundle.yaml

[Amanda Hager Lopes de Andrade Katz]

Goal:
Deploy bundle with existing machines set via bundle.yaml

Error:
ERROR cannot deploy bundle: the provided bundle has the following errors:
at least one application must be specified

Suggestion:
Give an error related to the machines validation.

Cause:
Set machines at bundle.yaml as:
name: openstack-base
series: focal
machines:
  '0': 1
  '1': 2
  '2': 3
applications:
  ceph-mon:
    annotations:
      gui-x: '790'
      gui-y: '1540'
    charm: ch:ceph-mon
    channel: quincy/stable
    num_units: 3
    options:
      expected-osd-count: 3
      monitor-count: 3
      source: cloud:focal-yoga
    to:
    - lxd:0
    - lxd:1
    - lxd:2
 ...

Entire bundle.yaml in:
https://pastebin.canonical.com/p/2XPxtDpprG/

Fix:
Set via parameter:
juju deploy ./bundle.yaml --map-machines=0=1,1=2,2=3

Repo:
https://github.com/juju/charm