fixup! Allow customization of input metadata delimiter

Use consistent csv.Error handling and add a comment on an assumption being made.

augur/io/metadata.py

@@ -168,8 +168,10 @@ def read_table_to_dict(table, valid_delimiters, duplicate_reporting=DataErrorMet
             # Note: this sort of duplicates _get_delimiter(), but it's easier if
             # this is separate since it handles non-seekable buffers.
             dialect = csv.Sniffer().sniff(table_sample, valid_delimiters)
-        except csv.Error:
-            raise InvalidDelimiter
+        except csv.Error as error:
+            # This assumes all csv.Errors imply a delimiter issue. That might
+            # change in a future Python version.
+            raise InvalidDelimiter from error
 
         metadata_reader = csv.DictReader(handle, dialect=dialect)
         if duplicate_reporting is DataErrorMethod.SILENT:
@@ -452,8 +454,7 @@ def _get_delimiter(path: str, valid_delimiters: Iterable[str]):
         try:
             # Infer the delimiter from the first line.
             return csv.Sniffer().sniff(file.readline(), valid_delimiters).delimiter
-        except csv.Error as err:
-            raise InvalidDelimiter(
-                f"Could not determine the delimiter of {path!r}. "
-                f"Allowed delimiters are: {valid_delimiters!r}"
-            ) from err
+        except csv.Error as error:
+            # This assumes all csv.Errors imply a delimiter issue. That might
+            # change in a future Python version.
+            raise InvalidDelimiter from error