Distinguish git worktree finding errors. (pantsbuild#21232)

Previously we would report an inability to locate the `git` binary as
"you're not in a git worktree", which is confusing.

Now we properly distinguish the cases in the user-facing error message.

src/python/pants/backend/python/util_rules/vcs_versioning.py

@@ -70,7 +70,7 @@ async def generate_python_from_setuptools_scm(
             softwrap(
                 f"""
                 Trying to determine the version for the {request.protocol_target.address} target at
-                {request.protocol_target.address}, but you are not running in a git worktree.
+                {request.protocol_target.address}, but {maybe_git_worktree.failure_reason}.
                 """
             )
         )

src/python/pants/vcs/git.py

@@ -255,6 +255,7 @@ def _parse_filename(self, match: re.Match) -> str | None:
 @dataclass(frozen=True)
 class MaybeGitWorktree(EngineAwareReturnType):
     git_worktree: GitWorktree | None = None
+    failure_reason: str | None = None  # If git_worktree is None, the reason why.
 
     def cacheable(self) -> bool:
         return False
@@ -272,7 +273,7 @@ async def get_git_worktree(
     maybe_git_binary: MaybeGitBinary,
 ) -> MaybeGitWorktree:
     if not maybe_git_binary.git_binary:
-        return MaybeGitWorktree()
+        return MaybeGitWorktree(failure_reason="couldn't find `git` binary")
 
     git_binary = maybe_git_binary.git_binary
     cmd = ["rev-parse", "--show-toplevel"]
@@ -284,8 +285,9 @@ async def get_git_worktree(
         else:
             output = git_binary._invoke_unsandboxed(cmd)
     except GitBinaryException as e:
-        logger.info(f"No git repository at {os.getcwd()}: {e!r}")
-        return MaybeGitWorktree()
+        failure_msg = f"no git repository at {os.getcwd()}: {e!r}"
+        logger.info(failure_msg)
+        return MaybeGitWorktree(failure_reason=failure_msg)
 
     git_worktree = GitWorktree(
         binary=git_binary,