server_version_from_bundle no longer raises error when it fails to detect the server version

## motivation

`server_version_from_bundle` was occasionally throwing warning or raising errors when it encountered executable directories.

### fixes

* now skips calling strings on executable directories
* instead of raising an error, it warns when it cannot deduce the server version from the bundle
* introduced SERVER_VERSION_NOT_AVAILABLE constant that @@server_version can be set to if the server_version_from_bundle fails to prevent repeated checks