Adds File existence check to Uploader

This handles the TODO for checking the existence of the files being
uploaded by the Uploader class.
In the case that a file does not exist, it raises an UploadError.

src/poetry/publishing/uploader.py

@@ -212,7 +212,6 @@ def _upload(
         self, session: requests.Session, url: str, dry_run: Optional[bool] = False
     ) -> None:
         for file in self.files:
-            # TODO: Check existence
 
             self._upload_file(session, url, file, dry_run)
 
@@ -225,6 +224,9 @@ def _upload_file(
     ) -> None:
         from cleo.ui.progress_bar import ProgressBar
 
+        if not file.is_file():
+            raise UploadError(f"Archive ({file}) does not exist")
+
         data = self.post_data(file)
         data.update(
             {

tests/publishing/test_uploader.py

@@ -60,3 +60,12 @@ def test_uploader_registers_for_appropriate_400_errors(mocker, http, uploader):
         uploader.upload("https://foo.com")
 
     assert 1 == register.call_count
+
+
+def test_uploader_properly_handles_file_not_existing(mocker, http, uploader):
+    mocker.patch("pathlib.Path.is_file", return_value=False)
+
+    with pytest.raises(UploadError) as e:
+        uploader.upload("https://foo.com")
+
+    assert f"Archive ({uploader.files[0]}) does not exist" == str(e.value)