Throwing a more descriptive warning when reading a bad .gitignore

A particularly unhelpful error was thrown before, I feel this will
be a lot more helpful to users trying to understand what went wrong.

src/black/files.py

@@ -122,7 +122,11 @@ def get_gitignore(root: Path) -> PathSpec:
     if gitignore.is_file():
         with gitignore.open(encoding="utf-8") as gf:
             lines = gf.readlines()
-    return PathSpec.from_lines("gitwildmatch", lines)
+
+    try:
+        return PathSpec.from_lines("gitwildmatch", lines)
+    except (IndexError, ValueError, TypeError) as e:
+        raise SyntaxError("Problem parsing .gitignore file: {e}")
 
 
 def normalize_path_maybe_ignore(

tests/data/broken_gitignore_test/.gitignore

@@ -0,0 +1,12 @@
+# No non-source python resources.
+*.pyc
+*.pyo
+*.pyd
+*.pyx
+
+# This should break things
+!
+
+# No build artifacts
+build
+wheelhouse

tests/test_black.py

@@ -1727,6 +1727,11 @@ def test_nested_gitignore(self) -> None:
         )
         self.assertEqual(sorted(expected), sorted(sources))
 
+    def test_broken_gitignore(self) -> None:
+        path = Path(THIS_DIR / "data" / "broken_gitignore_test")
+        with self.assertRaises(SyntaxError):
+            root_gitignore = black.files.get_gitignore(path)
+
     def test_empty_include(self) -> None:
         path = THIS_DIR / "data" / "include_exclude_tests"
         report = black.Report()