ModuleNotFoundError exception when IPython in not installed

[seisman]

python ~/bad.py (this is the script posted above)
Traceback (most recent call last):
  File "/Users/pwessel/bad.py", line 2, in <module>
    import pygmt
  File "/Users/pwessel/GMTdev/pygmt/pygmt/__init__.py", line 28, in <module>
    from pygmt.figure import Figure, set_display
  File "/Users/pwessel/GMTdev/pygmt/pygmt/figure.py", line 9, in <module>
    import IPython
ModuleNotFoundError: No module named 'IPython'

Originally posted by @PaulWessel in #732 (comment)

[seisman]

pygmt/pygmt/figure.py

Lines 8 to 11 in 7466dc3

	try:
	import IPython
	except KeyError:
	IPython = None # pylint: disable=invalid-name

When IPython is not installed, it raises a ModuleNotFoundError exception, but the code incorrectly checks the KeyError exception.

We should fix it and ideally add a test.