main: Open files with errors='replace'.

Stripe payment CSVs have apparently random encoding, and no way what to tell
what they're using beyond chardet.

The better way to fix this is following the existing to-do item, opening
files in binary mode and creating an importer base class to provide text
wrapping.  This would let each importer be as strict as possible.

import2ledger/__main__.py

@@ -51,7 +51,7 @@ class FileImporter:
     def import_path(self, in_path):
         if in_path is None:
             raise errors.UserInputFileError("only seekable files are supported", '<stdin>')
-        with in_path.open() as in_file:
+        with in_path.open(errors='replace') as in_file:
             if not in_file.seekable():
                 raise errors.UserInputFileError("only seekable files are supported", in_path)
             return self.import_file(in_file)