in lsl_get_dll.m, calls to websave should first check for local file

[cboulay]

A user in Slack encountered an error where the liblsl archive had already been downloaded. Maybe the download was interrupted or something else errored out before the library was extracted into the correct location.

Subsequent calls to lsl_get_dll sees that the library binary was missing so it attempts to download the archive again, but the archive is already present so this raises an error.

We can't handle all different error possibilities, so maybe the calls to websave should first check for the presence of the archive in the intended destination and if it's present then raise an error with a helpful message that they should try to download and unpack manually.