fontsetplot and ttfdecomponentizer fixes

[miguelsousa]

Fixes problems raised in #1125.

[josh-hadley]

In ttfdecomponentizer: this still fails if you specify -d + an input directory, but no output with -o. It seems like we are implying that if you specify a directory as input, you must also explicitly specify -o (and it must be a directory). But we aren't really enforcing that in the options validation.

Example:

ttfdecomponentizer -d some/input/directory -o some/output/directory

works, but

ttfdecomponentizer -d some/input/directory

fails (arg validation succeeds, but subsequent operations raise an Exception because the output path is None)

N.B. it also failed before these changes, but since we're in here, I think we should address it. Possible ways to handle this case (-d with valid input, but -o not specified):

1. set output directory == input. This means any files in the dir would get modified in-place, which is the default behavior of the tool in single-file mode
2. raise arg parser error something like, "must explicitly set -o when using -d for input". This is slightly "safer" than option 1 because it would require a user to intentionally set output dir == input dir.

[miguelsousa]

@josh-hadley good catch! Option 1 seemed the best to me, so I went with that.

[josh-hadley]

Awesome, thanks @miguelsousa !