Validation of the notebook existence before creating job or job def

[edwardps]

Problem

After selecting the "create job" button, the user is brought to the job creation UI with the input notebook path auto populated. After that there is no validation any more. If customer renames or removes the input notebook, there would be error - "Unexpected error occurred during creation of job." This error is not user friendly and actionable.

Proposed Solution

1. Add validation to prevent the customer from clicking "create" button if the notebook file does not exist (Preferred).
2. Or, show an actionable error message.