Don't reveal the real path on error msg

This is a recommit of #4317 which was meshed up!

#### Scope
Reaveal a trimmed path on an error message produced in /libraries/loader.php instead of the actual real path

#### B/C
None

#### Tests
Try renaming a library, so its unavailable. Loader will come up with an error 500 and the message 
```Library path /Applications/Mamp…. cannot be found.``` which is too liberal.
Apply the patch and re run
message should be ```Library path /libraries/... cannot be found.```