Handle exceptions gracefully

[felixvanoost]

Gracefully handles exceptions by exiting the Jekyll build process and displaying an error message. For instance, the stack trace displayed in #13 now shows up as:

[jekyll-kroki] Rendering diagrams using Kroki instance at 'https://expired.badssl.com/'
[jekyll-kroki] "SSL_connect returned=1 errno=0 peeraddr=104.154.89.105:443 state=error: certificate verify failed (certificate has expired)" in 'try_ssl_connect' on line 117 of /usr/local/rvm/gems/default/gems/httpx-1.1.1/lib/httpx/io/ssl.rb