Omit redundant 'Error in fail:' prefix from Starlark fail() error messages.

Calling `fail()` in Starlark previously formatted the final error line with `Error in fail: <message>` because `fail` is a builtin function and stack trace formatting prepended `Error in <builtin>: `. Because `fail()` is the standard mechanism in Starlark for rules, macros, and extensions to raise user-facing errors (such as invalid attribute values or unsupported targets), this prefix was redundant and made user errors look like internal failures within the `fail` function itself.

Format leaf `fail()` errors using the standard `Error: ` prefix, while preserving `Error in <builtin>: ` for other builtin functions (such as `getattr()`, `struct()`, or `dict()`).

For example, when a rule implementation calls `fail("unsupported platform: %s" % ctx.attr.platform)`:

Before:
```
Traceback (most recent call last):
	File "/workspace/pkg/rules.bzl", line 5, column 9, in _impl
		fail("unsupported platform: %s" % ctx.attr.platform)
Error in fail: unsupported platform: foo
```

After:
```
Traceback (most recent call last):
	File "/workspace/pkg/rules.bzl", line 5, column 9, in _impl
		fail("unsupported platform: %s" % ctx.attr.platform)
Error: unsupported platform: foo
```

RELNOTES: Starlark `fail()` calls now format error messages with `Error: ` instead of `Error in fail: `.
Resolves https://github.com/bazelbuild/bazel/issues/21523.
PiperOrigin-RevId: 971759352
Change-Id: Idda5352f05c8d7c297261ca65e4d2906369513a8