Handle io errors gracefully (#5611)

## Summary

It can happen that we can't read a file (a python file, a jupyter
notebook or pyproject.toml), which needs to be handled and handled
consistently for all file types. Instead of using `Err` or `error!`, we
emit E602 with the io error as message and continue. This PR makes sure
we handle all three cases consistently, emit E602.

I'm not convinced that it should be possible to disable io errors, but
we now handle the regular case consistently and at least print warning
consistently.

I went with `warn!` but i can change them all to `error!`, too.

It also checks the error case when a pyproject.toml is not readable. The
error message is not very helpful, but it's now a bit clearer that
actually ruff itself failed instead vs this being a diagnostic.

## Examples

This is how an Err of `run` looks now:


![image](890f7ab2-2309-4b6f-a4b3-67161947cc83)

With an unreadable file and `IOError` disabled:


![image](fd3d6959-fa23-4ddf-b2e5-8d6022df54b1)

(we lint zero files but count files before linting not during so we exit
0)

I'm not sure if it should (or if we should take a different path with
manual ExitStatus), but this currently also triggers when `files` is
empty:


![image](f7ede301-41b5-4743-97fd-49149f750337)

## Test Plan

Unix only: Create a temporary directory with files with permissions
`000` (not readable by the owner) and run on that directory. Since this
breaks the assumptions of most of the test code (single file, `ruff`
instead of `ruff_cli`), the test code is rather cumbersome and looks a
bit misplaced; i'm happy about suggestions to fit it in closer with the
other tests or streamline it in other ways. I added another test for
when the entire directory is not readable.

crates/ruff_shrinking/src/main.rs

@@ -429,7 +429,7 @@ fn main() -> ExitCode {
     if let Err(e) = run() {
         eprintln!("💥 Minimizer failed");
         for cause in e.chain() {
-            eprintln!("  Caused by: {cause}");
+            eprintln!("  Cause: {cause}");
         }
         ExitCode::FAILURE
     } else {