Detect that Stash is unable to execute its hooks scripts and do something about it

Currently when Stash's hook scripts for a given repo are missing that fact is silently ignored and as a result all the plugin hooks configured for the repo will not execute.

I am not sure about other scenarios (e.g. no exec permission etc.), but generally the fact that the hooks were unable to execute must be detected and dealt with somehow, e.g.:

• big fat ERROR in logs
• notifying admin somehow
• warning message on the hook settings page
• whatever else to make sure the admin is aware of this (and perhaps link/suggestion to raise a support request)