In batch mode, check if urls point to valid files in the batch job itself instead of the subjobs

[pbordron]

Describe the bug

When using incorrect url in batch file, url is reported as wrong when the subjob starts instead when submitting the batch job.
An email is send when the job fails, but the reason is missing as there is not logs for this subjob (the subjob hesn't started)
The expected error message is only displayed in status page.

Behavior observed

• If the url point to a file that exists but has an incorrect type (like a folder), the error message append at batch job level and no error email is sent.
• If the url point to a file that does not exist, the error message append at subjob level, and an error email is sent.

Expected behavior
When clicking on submit, the file existence pointed by url must be checked and return an error message if incorrect