Outputting to a directory instead of an existing file raises an exception

[emeryberger]

Describe the bug
Using an existing directory as the argument for --outfile (this was unintentional) led to an internal exception.

To Reproduce
Steps to reproduce the behavior:

1. Create a directory foo
2. Run Scalene as follows on any sufficiently long-lived Python program: scalene --cli --outfile foo prog.py
3. See error, which will end with something like this:

IsADirectoryError: [Errno 21] Is a directory: '/Users/XXX/tmp/foo'

Minimum working example:
prog.py:

x = 1
for i in range(1_000_000):
    x += 1
print("Hello world")

Expected behavior
A more graceful exit. It might also be reasonable to check to make sure there is not an existing directory when the command is first launched.