traceback that is seen if casapy is not present as an alias to the CASA.app

[trhunter]

[oakenshield:~/Downloads] thunter% python setup_casapy_pip.py
Traceback (most recent call last):
  File "setup_casapy_pip.py", line 222, in <module>
    python_version = get_python_version_mac()
  File "setup_casapy_pip.py", line 42, in get_python_version_mac
    casapy_path = get_casapy_path()
  File "setup_casapy_pip.py", line 38, in get_casapy_path
    return os.path.realpath(find_executable('casapy'))
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.py", line 367, in realpath
    path, ok = _joinrealpath('', filename, {})
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.py", line 373, in _joinrealpath
    if isabs(rest):
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.py", line 61, in isabs
    return s.startswith('/')
AttributeError: 'NoneType' object has no attribute 'startswith'
[oakenshield:~/Downloads] thunter% which casapy
casapy: Command not found.

[keflavich]

The problem is that casapy isn't found; we should catch this error and report it sensibly. This can happen even if /usr/bin/casapy exists if it is a symlink to a nonexistent location. This bug has occurred at least once (in @trhunter's case) because CASA.app was first run from within the DMG-mounted filesystem (i.e., the thing that comes up if you double-click on a dmg file).