Improve iOS Runner build failure reporting

[belemaire]

• Return a Promise rather than void from launchIosRunner function so that in case of an error being thrown we don't end up with an ugly UnhandledPromiseRejectionWarning.

• Improve error message in case of iOS Runner build failure, by giving some suggestion to the user to help troubleshoot the build issue further (rather than just facing a bare xcbuild error code without any clue of what to do next).

Before

(node:31119) UnhandledPromiseRejectionWarning: Error: XCode xcbuild command failed with exit code 65.
    at ChildProcess.xcodebuildProc.on.code (/Users/blemair/Code/electrode-native/ern-orchestrator/src/buildIosRunner.ts:33:30)
    at emitTwo (events.js:126:13)
    at ChildProcess.emit (events.js:214:7)
    at maybeClose (internal/child_process.js:925:16)
    at Socket.stream.socket.on (internal/child_process.js:346:11)
    at emitOne (events.js:116:13)
    at Socket.emit (events.js:211:7)
    at Pipe._handle.close [as _onclose] (net.js:557:12)
(node:31119) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 3)
(node:31119) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

After

✖ An error occurred: iOS Runner build failed [xcbuild exit code 65].
✖ To troubleshoot this build failure, we recommend building the Runner iOS project from XCode.
✖ You can open the Runner project in XCode manually or by running 'open ios/ErnRunner.xcodeproj'.
✖ Building the Runner from XCode will provide more meaningful error reporting that can be of help
✖ to pinpoint the cause of the build failure.