fix: warn user about bad pattern paths before skipping step git

[SteveLauC]

Standards checklist:

• The PR title is descriptive.
• I have read CONTRIBUTING.md
• The code compiles (cargo build)
• The code passes rustfmt (cargo fmt)
• The code passes clippy (cargo clippy)
• The code passes tests (cargo test)
• Optional: I have tested the code myself
  • I also tested that Topgrade skips the step where needed

If you developed a feature or a bug fix for someone else and you do not have the
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---

We should warn the user about the bad pattern before skipping the step git:

$ cat ~/.config/topgrade/topgrade.toml
[git]
pull_predefined = false
repos = [
    "bad_pattern"
]

# No warning at all because all my configured paths are invalid, and thus 
# `Repositories.repositories` `is_empty()`, the git step is skipped
$ topgrade --dry-run --only git_repos
$ echo $?
0

$ ./target/debug/topgrade --dry-run --only git_repos
Path bad_pattern did not contain any git repositories

---

I think besides Skipped, we should define another state NotFound, when a step's binary is not found in $PATH, we use NotFound to skip this step instead of Skipped. For the case where the binary is found but some conditions are not satisfied during execution (e.g., for git step, no repo is found, like the case described in this PR), we use Skipped.

And for a Skipped step, a separator should be printed and a warning should be given to the user telling the user why this step was skipped. For a NotFound step, we don't need to print a separator as users don't use it at all.