Add dialog.showErrorBox API
win: Implement dialog.showErrorBox
Use dialog.showErrorBox for showing errors
mac: Implement dialog.showErrorBox
linux: Print error to console when GUI is not ready
Don't print error to console in default_app
linux: Use GTK+ for error reporting when GUI is ready
Forbid using dialog module before app is ready
Merge pull request #790 from atom/error-box

Fix crash when showing error dialog before GUI environment is initialized