Update error message for invalid python_path

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: mai93

python/private/py_executable.bzl

@@ -1144,7 +1144,7 @@ def _get_runtime_details(ctx, semantics):
 
     flag_interpreter_path = read_possibly_native_flag(ctx, "python_path")
     if not flag_interpreter_path.startswith("python") and not paths.is_absolute(flag_interpreter_path):
-        fail("python_path must be an absolute path when it is set.")
+        fail("'python_path' must be an absolute path or a name to be resolved from the system PATH (e.g., 'python', 'python3').")
 
     toolchain_runtime, effective_runtime = _maybe_get_runtime_from_ctx(ctx)
     if not effective_runtime: