Raise on invalid file path

[Sija]

Resolves #787
Closes #388

Adds --ignore-unmatched-paths CLI flag to opt out of the new behaviour.

Original description

The intent of this PR is to raise if any of the provided inputs resolve to no files.

Before (silent success — misleading):

# Typo in filename — ameba says "all good!" but checked nothing
$ ameba typo.cr # => exits 0, no errors reported

# Non-existent directory — same problem
$ ameba "src/nonexistent/**/*.cr" # => exits 0, no errors reported

After (clear error):

$ ameba typo.cr
# => raises: "No files found matching `typo.cr`"

$ ameba "src/nonexistent/**/*.cr"
# => raises: "No files found matching `src/nonexistent/**/*.cr`"

[straight-shoota]

FWIW: When running the command ameba src/nonexistent/**/*.cr in a shell, the shell typically already expands the glob pattern and errors if it doesn't match.

This patch only has an effect when ameba resolves the glob pattern instead of the shell.
The command ameba 'src/nonexistent/**/*.cr' would be better for the change description. The quoting makes sure ameba receives the original pattern instead of its expansion.

[veelenga]

lgtm 👍🏻