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

Original issue: https://github.com/uber/RIBs/issues/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.

Start with ios/tooling/install-xcode-template.sh and reproduce the reported permission-denied case against an Xcode installation whose template directory is not writable. Trace how mkdir and cp failures are handled; done means the script reports failure and does not print success when setup did not complete.