gracefully test if provider is up

[siebrenf]

UCSC (http://hgdownload.soe.ucsc.edu/) is down today. when calling UCSC, genomepy simply hangs. As @Maarten-vd-Sande suggested, it would be great if this is caught instead.

status check
The test would check a provider-specific url, and fail if a negative/no response is obtained after ~10 seconds or so. In a perfect world, this is only checked when going online, but since that is most of genomepy, it could be checked on provider init as well. Extra points if this is added to a 4 hour bucketcache per provider.

integration
Function that cycle over all providers (search) could even skip these providers with a warning.