Validate existence of iOS SDK and simulators

[olgamirth]

Unless an Apple Simulator Control Device is installed, a 'briefcase build iOS' will fail. It can be determined from the command xcrun simctl list devices. A successful output will return a number of iOS devices.

I added a "try" routine to check if the Apple Simulator Control devices are installed. If just the line
"== Devices ==" is returned, an exception is raised and a helpful message is added to the Preamble

The problem is described in Issue 2286. Unless the full Xcode app along with the iOS SDK is installed,
a briefcase build iOS will fail.

Fixes #2286

PR Checklist:

• All new features have been tested
• All new features have been documented
• [X ] I have read the CONTRIBUTING.md file
• [X ] I will abide by the code of conduct

[freakboy3742]

@olgamirth Just checking in - are you planning to continue working on this, or can we release the issue for someone else to work on it?

[olgamirth]

@freakboy3742 ... so sorry for not responding sooner. It got lost in the shuffle of things.

If you haven't reassigned this task to someone else, I would very much like to finish working on it. I'll wait to hear from you before I add any commits to make sure I'm not "stepping on any toes"

Thanks ... and again, my sincere apologies.
Mike

[freakboy3742]

@freakboy3742 ... so sorry for not responding sooner. It got lost in the shuffle of things.

No need to apologize - I was just checking in to see if we could release this issue for someone else to look at. If you're still working on it (or planning to), that's no problem - I'll direct people somewhere else.