exception handling for googletrans

• improved exception handling for googletrans
• skip translation if error occurs, print warning and move on #237
• constrain the maximum number of characters in the extracted text to 5000, as this is the limit for googletrans #240
• please note that this constraints the original text to be truncated to 5000 characters too, same string length as the translated text, to keep the 1:1 correspondence of original and translated text. It would be possible though to store the full length original, I do not see the use case for this now, but could be added as an (optional) feature