Give a clearer error when Safari Technology Preview isn't installed

[gsnedders]

By default you get something like:

gsnedders@gsnedders-margot web-platform-tests % ./wpt run safari foo 
/Volumes/gsnedders/projects/wpt/web-platform-tests/_venv3/lib/python3.9/site-packages/urllib3/__init__.py:35: NotOpenSSLWarning: urllib3 v2 only supports OpenSSL 1.1.1+, currently the 'ssl' module is compiled with 'LibreSSL 3.3.6'. See: https://github.com/urllib3/urllib3/issues/3020
  warnings.warn(
Traceback (most recent call last):
  File "/Volumes/gsnedders/projects/wpt/web-platform-tests/./wpt", line 10, in <module>
    wpt.main()
  File "/Volumes/gsnedders/projects/wpt/web-platform-tests/tools/wpt/wpt.py", line 233, in main
    rv = script(*args, **kwargs)
  File "/Volumes/gsnedders/projects/wpt/web-platform-tests/tools/wpt/run.py", line 994, in run
    setup_cls, wptrunner_kwargs = setup_wptrunner(venv, **kwargs)
  File "/Volumes/gsnedders/projects/wpt/web-platform-tests/tools/wpt/run.py", line 966, in setup_wptrunner
    setup_cls.setup(kwargs)
  File "/Volumes/gsnedders/projects/wpt/web-platform-tests/tools/wpt/run.py", line 248, in setup
    self.setup_kwargs(kwargs)
  File "/Volumes/gsnedders/projects/wpt/web-platform-tests/tools/wpt/run.py", line 740, in setup_kwargs
    raise WptrunError("Unable to locate safaridriver binary")
tools.wpt.run.WptrunError: Unable to locate safaridriver binary

This is a pretty bad user experience; it's not at all obvious what it was looking for (/Applications/Safari Technology Preview.app/Contents/MacOS/safaridriver).