Error Loading Certs from Directory

It appears like there is no way to exclude a subdirectory when trying to load certs from a specific path.

I am running into an issue using the `vault-ssh-helper` and think this is the root cause.

Ideally, [this line](https://github.com/hashicorp/go-rootcerts/blob/63503fb4e1eca22f9ae0f90b49c5d5538a0e87eb/rootcerts.go#L91) shouldn't be erroring IMO, rather it should be throwing a warning.

Alternatively, a way to specify specific files/dirs to exclude when calling [`LoadCAPath`](https://github.com/hashicorp/go-rootcerts/blob/63503fb4e1eca22f9ae0f90b49c5d5538a0e87eb/rootcerts.go#L73) would help resolve this issue too.

Any advice would be much appreciated