Don't use internal templates anymore.

Blag will throw an error if a template is not found locally. The error
message contains an explanation for the user on where to get the missing
templates.

Quickstart will generate templates for the user in the working
directory.

Updated tests appropriately.

tests/test_blag.py

@@ -179,9 +179,9 @@ author = a. u. thor
         assert config_parsed['base_url'] == 'https://example.com/'
 
 
-def test_environment_factory() -> None:
+def test_environment_factory(cleandir: str) -> None:
     globals_: dict[str, object] = {'foo': 'bar', 'test': 'me'}
-    env = blag.environment_factory(globals_=globals_)
+    env = blag.environment_factory("templates", globals_=globals_)
     assert env.globals['foo'] == 'bar'
     assert env.globals['test'] == 'me'
 
@@ -299,6 +299,22 @@ foo bar
     assert os.path.exists(f'{args.output_dir}/tags/bar.html')
 
 
+@pytest.mark.parametrize(
+    'template',
+    [
+        'page.html',
+        'article.html',
+        'archive.html',
+        'tags.html',
+        'tag.html',
+    ]
+)
+def test_missing_template_raises(template: str, args: Namespace) -> None:
+    os.remove(f'templates/{template}')
+    with pytest.raises(SystemExit):
+        blag.build(args)
+
+
 def test_main(cleandir: str) -> None:
     blag.main(['build'])