[Port] Better error message for slash in hostname

[jlord]

This ports #1559 from the 2.2 branch to 3.0.

Notes:

• There were changes between 2.2 and 3.0 in the test set up so it's not a direct transfer there.
• I had to add the check && r.path.indexOf('::') === -1 to line 203 because without it both test URIs end up with the slash error because they each contain 3 items in their array after the split:

# mongodb://invalid::host:27017/db
path split [ '', '::host', 'db' ]
# mongodb://invalid/host:27017/db
path split [ '', 'host:27017', 'db' ]

I'm not quite sure why the tests passed without this in 2.2 cc @mbroadst for thoughts 💭