[2.7] improve errors when resolving bundle includes

[achilleasa]

Description of change

This PR brings in the changes from juju/charm#301 so the juju CLI can display more informative error messages when failing to locate a relative include from a base bundle or an overlay.

QA steps

$ mkdir bananas ; cd bananas

#----------------------
# Check error for a base bundle include
#----------------------
$ cat > bundle.yaml <<EOT
applications:
  containerd:
    charm: cs:~containers/containerd-53
    options:
      foo: include-file://../secrets/secret-password.txt
EOT

$ juju deploy ./bundle.yaml --dry-run
ERROR cannot deploy bundle: processing option "foo" for application "containerd": resolving include "../secrets/secret-password.txt": include file "$PWD/secrets/secret-password.txt" not found

#----------------------
# Check error for an overlay include
#----------------------
$ cat > bundle.yaml <<EOT
applications:
  containerd:
    charm: cs:~containers/containerd-53
EOT

$ mkdir bar
$ cat > bar/overlay.yaml <<EOT
applications:
  containerd:
    charm: cs:~containers/containerd-53
    options:
      foo: include-file://../secrets/secret-password.txt
EOT

# Note that the absolute path in the following error has been resolved relative to the *overlay* path
$ juju deploy ./bundle.yaml --overlay bar/overlay.yaml --dry-run
ERROR cannot deploy bundle: processing option "foo" for application "containerd": resolving include "../secrets/secret-password.txt": include file "$PWD/bananas/secrets/secret-password.txt" not found

# Compare the above errors to the ones generated by an older 2.7 CLI (e.g. from snap)

Bug reference

Improves error messages as discussed in https://bugs.launchpad.net/juju/+bug/1851817

[achilleasa]

$$merge$$