Upload of app with invalid archive fails with 500

[MorrisJobke]

The UI doesn't reflect if there happens a 500 during upload of an invalid archive:

Traceback (most recent call last):
  File "/usr/lib/python3.5/tarfile.py", line 1629, in gzopen
    t = cls.taropen(name, mode, fileobj, **kwargs)
  File "/usr/lib/python3.5/tarfile.py", line 1605, in taropen
    return cls(name, mode, fileobj, **kwargs)
  File "/usr/lib/python3.5/tarfile.py", line 1470, in __init__
    self.firstmember = self.next()
  File "/usr/lib/python3.5/tarfile.py", line 2279, in next
    tarinfo = self.tarinfo.fromtarfile(self)
  File "/usr/lib/python3.5/tarfile.py", line 1082, in fromtarfile
    buf = tarfile.fileobj.read(BLOCKSIZE)
  File "/usr/lib/python3.5/gzip.py", line 274, in read
    return self._buffer.read(size)
  File "/usr/lib/python3.5/_compression.py", line 68, in readinto
    data = self.read(len(byte_view))
  File "/usr/lib/python3.5/gzip.py", line 461, in read
    if not self._read_gzip_header():
  File "/usr/lib/python3.5/gzip.py", line 409, in _read_gzip_header
    raise OSError('Not a gzipped file (%r)' % magic)
OSError: Not a gzipped file (b'us')

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/appstore/venv/lib/python3.5/site-packages/django/core/handlers/exception.py", line 35, in inner
    response = get_response(request)
  File "/opt/appstore/venv/lib/python3.5/site-packages/django/core/handlers/base.py", line 128, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/opt/appstore/venv/lib/python3.5/site-packages/django/core/handlers/base.py", line 126, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/opt/appstore/venv/lib/python3.5/site-packages/django/views/decorators/csrf.py", line 54, in wrapped_view
    return view_func(*args, **kwargs)
  File "/opt/appstore/venv/lib/python3.5/site-packages/django/views/generic/base.py", line 69, in view
    return self.dispatch(request, *args, **kwargs)
  File "/opt/appstore/venv/lib/python3.5/site-packages/rest_framework/views.py", line 483, in dispatch
    response = self.handle_exception(exc)
  File "/opt/appstore/venv/lib/python3.5/site-packages/rest_framework/views.py", line 443, in handle_exception
    self.raise_uncaught_exception(exc)
  File "/opt/appstore/venv/lib/python3.5/site-packages/rest_framework/views.py", line 480, in dispatch
    response = handler(request, *args, **kwargs)
  File "/opt/appstore/nextcloudappstore/api/v1/views.py", line 181, in post
    info, data = provider.get_release_info(url, is_nightly)
  File "/opt/appstore/nextcloudappstore/api/v1/release/provider.py", line 34, in get_release_info
    meta = self.extractor.extract_app_metadata(download.filename)
  File "/opt/appstore/nextcloudappstore/api/v1/release/parser.py", line 74, in extract_app_metadata
    with tarfile.open(archive_path, 'r:gz') as tar:  # type: ignore
  File "/usr/lib/python3.5/tarfile.py", line 1575, in open
    return func(name, filemode, fileobj, **kwargs)
  File "/usr/lib/python3.5/tarfile.py", line 1633, in gzopen
    raise ReadError("not a gzip file")
tarfile.ReadError: not a gzip file
Internal Server Error: /api/v1/apps/releases

cc @BernhardPosselt @adsworth

Found by @violoncelloCH in nextcloud/user_external#26 (comment)

[MorrisJobke]

And by @skjnldsv today

[BernhardPosselt]

Ah, should be a 400