Print minimal help text if we can't connect to the server

If the client could not connect to the server for whatever reason, we used
to spew an uninformative backtrace. Now, we print a more helpful message.
Print a helpful message on SocketError

When running something like 'razor nodes', we used to just spew a
backtrace. Print a slightly more friendly error message.
Allow setting the connection URL with the RAZOR_API environment variable

This should make usage from the command line against a server that is not
local more convenient.