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.

Author: Pradyumna Rahul

Diff for: src/poetry/publishing/uploader.py

@@ -212,7 +212,10 @@ def _upload(
         self, session: requests.Session, url: str, dry_run: Optional[bool] = False
     ) -> None:
         for file in self.files:
-            # TODO: Check existence
+            if not file.is_file():
+                raise UploadError(
+                    "Wheel or Tar files associated with the Project Package do not exist"
+                )
 
             self._upload_file(session, url, file, dry_run)
 

Diff for: tests/publishing/test_uploader.py

@@ -60,3 +60,14 @@ 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 "Wheel or Tar files associated with the Project Package do not exist" == str(
+        e.value
+    )