[ios/tooling] the install-xcode-template fails if file permissions aren't exactly as expected

[alexvbush]

Original issue: uber/RIBs#293

It looks like the bash script has no error handling, so when mkdir fails the script isn't able to copy over the templates. Due to the lack of error handling the script still thinks it succeeded

👋 ./ios/tooling/install-xcode-template.sh (🙈) ~/Developer/ribs💻
==> Copying up RIBs Xcode file templates...
mkdir: /Applications/xcode_10_fb.app/Templates/File Templates/RIBs: Permission denied
usage: cp [-R [-H | -L | -P]] [-fi | -n] [-apvXc] source_file target_file
cp [-R [-H | -L | -P]] [-fi | -n] [-apvXc] source_file ... target_directory
usage: cp [-R [-H | -L | -P]] [-fi | -n] [-apvXc] source_file target_file
cp [-R [-H | -L | -P]] [-fi | -n] [-apvXc] source_file ... target_directory
usage: cp [-R [-H | -L | -P]] [-fi | -n] [-apvXc] source_file target_file
cp [-R [-H | -L | -P]] [-fi | -n] [-apvXc] source_file ... target_directory
==> ... success!
==> RIBs have been set up. In Xcode, select 'New File...' to use RIBs templates.