Re enabled gevent as dependency

Fix #1399 (Calibre-web starting with installed and not activated gdrive denpendencies without internet connection again)

cps/gdriveutils.py

@@ -33,6 +33,7 @@ try:
     from pydrive.drive import GoogleDrive
     from pydrive.auth import RefreshError
     from apiclient import errors
+    from httplib2 import ServerNotFoundError
     gdrive_support = True
 except ImportError:
     gdrive_support = False
@@ -192,9 +193,13 @@ def getDrive(drive=None, gauth=None):
     return drive
 
 def listRootFolders():
-    drive = getDrive(Gdrive.Instance().drive)
-    folder = "'root' in parents and mimeType = 'application/vnd.google-apps.folder' and trashed = false"
-    fileList = drive.ListFile({'q': folder}).GetList()
+    try:
+        drive = getDrive(Gdrive.Instance().drive)
+        folder = "'root' in parents and mimeType = 'application/vnd.google-apps.folder' and trashed = false"
+        fileList = drive.ListFile({'q': folder}).GetList()
+    except ServerNotFoundError as e:
+        log.info("GDrive Error %s" % e)
+        fileList = []
     return fileList