Current git master does not import cleanly if bottleneck is not installed

[lbeltrame]

The culprit is in nanops.py, line 17 (some extra lines shown for context):

try:
    import bottleneck as bn
    _USE_BOTTLENECK = True
except ImportError:  # pragma: no cover
    _USE_BOTTLENECK = False

def _bottleneck_switch(bn_name, alt, **kwargs):
    try:
        bn_func = getattr(bn, bn_name)

bn is only defined if bottleneck is imported and the try statement doesn't catch the NameError that ensues (only AttributeError).

[wesm]

Related to a change I made minutes ago. Catching both AttributeError and NameError now