tree a0c0cc8a73a5aa98208c430e33584eea88ce244d
parent 1a9fc6149393d0e9c175318e456660cef73fb327
author Neri Marschik <nerima@google.com> 1765932463 +0900
committer GN LUCI <gn-scoped@luci-project-accounts.iam.gserviceaccount.com> 1767746931 -0800

Improve error message for rebase_path builtin

When rebase_path encounters an invalid path (e.g., an empty string), it
previously blamed the origin of the value. If the value originated from
another function (like read_file) or a variable definition far away,
the error message could be confusing as it pointed to the definition
site rather than the usage site.

This change improves the error reporting by:
- Rewriting the error to blame the rebase_path call.
- Appending the original error location as a sub-error. This makes it clear that
rebase_path failed, while still providing context on where the bad value came
from.

Before
```
ERROR at //BUILD.gn:2:9: Empty directory path.
lines = read_file("test.txt", "list lines")
	^---------------------------------
You can't use empty strings as directories.
```

After
```
ERROR at //BUILD.gn:4:13: Empty directory path.
rebase_path(lines)
	    ^----
You can't use empty strings as directories.
See //BUILD.gn:3:9: The value that caused the error.
lines = read_file("test.txt", "list lines")
	^
```

Change-Id: I7c5d26ee13ac5fea3560a808a9d4262a0b9304fc
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/20660
Reviewed-by: David Turner <digit@google.com>
Commit-Queue: Neri Marschik <nerima@google.com>