configure_endpoint_function -- check for valid 'xtract_path' argument

[jvsr1]

We should verify that the user provides a valid path on the funcx endpoint when they configure their endpoint via the 'configure_ep/<funcx_eid>' route.

Essentially there are several requirements:

1. Ensure that the path is valid (either exists or can be created)
2. Ensure that the user has valid permissions
3. Correct syntax

Please comment if there any more requirements that should be added to this list.