Better error messages

[jkuchar]

Feature request

When there is any error happening on the xcrun simctl tool, there is no way how to get it know in the test (in the log). When one do not know the tool well, one ends up with the following terminal output, which is not useful at all:

Currently there is no way you can retrieve error message that has bee produced by xcrun simctl tool.

Describe the solution you'd like

Forward the same output that is available from --verbose to HTTP body response and make it available in Error.

Describe alternatives you've considered

There can be also some kind of instructions on the client side, to turn on --verbose mode od the server and run it in some visible terminal.

Additional context

I have found this while was not able to find out why my simple UI test does not work and found out, that it was related to #19 parallel tests (resulted in draft of fix #21 )