Make more informative warning message when failed to parse pyproject.toml (#6009)

## Summary

Added the actual error message to the warning when uv fails to parse
`pyproject.toml`.

Resolves https://github.com/astral-sh/uv/issues/5934

## Test Plan

Took the case from the issue:
- have `pyproject.toml` which contains
```
[tool.uv]
foobar = false
```
- 
```
$ uv venv --preview -v
```
- Expect the message that contains the actual problem in the
`pyproject.toml` like:
```
warning: Failed to parse `pyproject.toml` during settings discovery: unknown field `foobar`; skipping...
```

crates/uv-settings/src/lib.rs

@@ -72,9 +72,13 @@ impl FilesystemOptions {
                 Ok(None) => {
                     // Continue traversing the directory tree.
                 }
-                Err(Error::PyprojectToml(file, _err)) => {
+                Err(Error::PyprojectToml(file, err)) => {
                     // If we see an invalid `pyproject.toml`, warn but continue.
-                    warn_user!("Failed to parse `{file}` during settings discovery; skipping...");
+                    warn_user!(
+                        "Failed to parse `{}` during settings discovery:\n{}",
+                        file.cyan(),
+                        textwrap::indent(&err.to_string(), "  ")
+                    );
                 }
                 Err(err) => {
                     // Otherwise, warn and stop.