Make the NO_INSTALL_PREFIX_MESSAGE error message include Python-only tip

[dhermes]

From #275 (and many other bug reports are similar):

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Collecting bezier
  Using cached bezier-2021.2.12.tar.gz (313 kB)
    ERROR: Command errored out with exit status 1:
     command: /usr/bin/python3 -c 'import io, os, sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-c7l04qpk/bezier_1ae222099e174d0db3a03720f989db8b/setup.py'"'"'; __file__='"'"'/tmp/pip-install-c7l04qpk/bezier_1ae222099e174d0db3a03720f989db8b/setup.py'"'"';f = getattr(tokenize, '"'"'open'"'"', open)(__file__) if os.path.exists(__file__) else io.StringIO('"'"'from setuptools import setup; setup()'"'"');code = f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-pip-egg-info-bdebbu8f
         cwd: /tmp/pip-install-c7l04qpk/bezier_1ae222099e174d0db3a03720f989db8b/
    Complete output (1 lines):
    The BEZIER_INSTALL_PREFIX environment variable must be set.
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Used exactly once:

bezier/setup.py

Line 127 in 2143861

print(NO_INSTALL_PREFIX_MESSAGE, file=sys.stderr)