Catch JSON decoding errors in browser.get

See: https://progress.opensuse.org/issues/100709

openqa_review/browser.py

@@ -148,15 +148,22 @@ def _get(self, url, as_json=False):  # pragma: no cover
             msg = "Request to {} was not successful: {}".format(url, str(e))
             log.warn(msg)
             raise DownloadError(msg)
+        return self.decode_content(r, url, as_json)
 
+    def _decode_content(self, url, r, as_json=False):  # pragma: no cover
         try:
             r.raise_for_status()
         except requests.exceptions.HTTPError as e:
             msg = "Request to {} failed: {}".format(url, str(e))
             log.warn(msg)
             raise DownloadError(msg)
 
-        content = r.json() if as_json else r.content.decode("utf8")
+        try:
+            content = r.json() if as_json else r.content.decode("utf8")
+        except json.decoder.JSONDecodeError as e:
+            msg = 'Unable to decode JSON for {}: {} (Content was: "{}")'.format(url, str(e), r.content.decode("utf8"))
+            log.warn(msg)
+            raise DownloadError(msg)
         return content
 
     def json_rpc_get(self, url, method, params, cache=True):