Include proper error handling and warnings

[IQAndreas]

Allow for better error handling for the following cases:

• There is a problem with the config options
• The GitHub API returns an error
• The target issue already exists in the repository (should at least warn)

I also want to prompt the user "You are about to import all 42 issues into hello-world. Are you sure you want to continue?"

[IQAndreas]

@dliuzzi has already done this for warnings thrown by the GitHub API: https://github.com/dliuzzi/tools

All that needs to be done is filter through the commits and make the changes (I'm pretty sure this code is too far-gone to do a straight up merge).

[IQAndreas]

Fixed in the following commits:

• Add proper error handling for HTTP requests
• Add error if the config file cannot be found
• Add error if missing required config options