Throw proper error when nightwatch config file not found.

[garg3133]

In Nightwatch VSCode extension, if the nightwatch.conf.js file is not found, an error is thrown which gives out no information on what went wrong. This PR aims to throw a proper error instead.

PR not tested yet.

[vaibhavsingh97]

Instead we can show the popup window to show the error with the path

[garg3133]

Parking it for the moment as the implementation seems to be incomplete.

If a config file exists then require shouldn't throw an error (or maybe it will if the config has some wrong JS syntax?). But the main issue we want to encounter here is if nightwatch.conf.js does not exist, then the user should get a proper error message, but then we should also show a proper error message if the syntax/format of the config file is not right. See: #42

Note that if no config file is present, then res will be an empty array, so we should throw some error for the user in that case. Also, if a config file is present but the syntax is not right, then also we should throw some error.