feat(webdriver-manager): minor proxy enhancements

Added error handling for request - previously, any errors coming from
the request module were silently swallowed.

Also evaluating both uppercase and lowercase proxy variables. Many tools
use proxy variables in the form https_proxy, others use HTTPS_PROXY. I
changed the code so both forms are evaluated. See the following links
for more info on this practice:

* http://serverfault.com/q/571887/140074
* https://wiki.archlinux.org/index.php/proxy_settings#Environment_variables