Add error handling for checking if the machine doesn't have a command to open browser

[devhindo]

Extracted from #46

Error Message:

./x auth
panic: exec: "xdg-open": executable file not found in $PATH

goroutine 1 [running]:
github.com/devhindo/x/src/cli/utils.OpenBrowser(...)
/home/devhindo/gh/x/src/cli/utils/open.go:11
github.com/devhindo/x/src/cli/auth.(*User).open_browser_to_auth_url(...)
/home/devhindo/gh/x/src/cli/auth/user.go:45
github.com/devhindo/x/src/cli/auth.Auth()
/home/devhindo/gh/x/src/cli/auth/auth.go:18 +0xd6
github.com/devhindo/x/src/cli/x.HandleArgs()
/home/devhindo/gh/x/src/cli/x/args.go:28 +0x2e5
github.com/devhindo/x/src/cli/x.Run(...)
/home/devhindo/gh/x/src/cli/x/run.go:4
main.main()
/home/devhindo/gh/x/src/cli/main.go:7 +0x10

• Add validations to check if linux doesn't have xdg-open command. #51
• Add validations to check if windows doesn't have rundll32 command. #54
• Add validations to check if darwin doesn't have open command. #52
• make default behaviour to output the URL and prompt the user to validate manually. #53

Can't open your browser, please validate using your favorite browser.