HTTP code 500 is returned instead of 404 when wrong manifest name is send to stack analysis

[tisnik]

Current behavior:

when used sends an improper manifest name (requirements.2.txt instead of requirements.txt) to the stack analysis endpoint, the server responds by:

<Response [500]>
{'error': 'Could not process a29d90f4d8a8470a98e130b33fd4dc74.'}

Expected behavior:

• parameters to be checked at the beginning of processing
• HTTP code should be 400 Bad Request not 500 Internal Server Error
• error message should include the root cause