Give user-friendly error messages when things go wrong rather than a stack trace

[cdfarrow]

Two scenarios hit me regularly, and the standard Python error trace is just ugly and makes it difficult to quickly see what the problem is:

1. I often don't have internet turned on and get a whole barrage of fails "WARNING: Retrying... Failed to establish a new connection" finally finishing with a useless traceback. This is an easily anticipated scenario, which should be caught and the user informed nicely. [BTW, I just found out about the -n option and have started using that; I don't know why you'd want to download setuptools, etc. every time you run build.]
2. I often open up the .whl file in a zip viewer to verify/review the contents. If I run build again without closing it, I get the traceback with "PermissionError: [WinError 32] The process cannot access the file because it is being used by another process". Again, this is very unfriendly and yet a simple scenario to trap and give a friendly one-line message instead of a screenful of stack trace.
   EDIT: I just realised that the second issue is with setuptools, not wheel (sorry), so I'll report this in that repo, too. Overall, though, stack dumps seem to be a regular part of using the Python build tools, and besides being difficult to interpret, I'd suggest that they give an unprofessional impression.