imagej.doctor should notice subprocess crashes

[ctrueden]

From #267, the imagej.doctor checkup printed:

Checking Maven:
--> Maven executable = /Users/brandonulin/anaconda3/envs/pyimagej/bin/mvn
$ mvn -v
Command '['/Users/brandonulin/anaconda3/envs/pyimagej/bin/mvn', '-v']' died with <Signals.SIGKILL: 9>.

Checking Java:
--> JAVA_HOME = /Users/brandonulin/anaconda3/envs/pyimagej
--> Java executable = /Users/brandonulin/anaconda3/envs/pyimagej/bin/java
$ java -version
Command '['java', '-version']' died with <Signals.SIGKILL: 9>.

Great job! All looks good.

Both the mvn and java subprocess calls crashed with SIGKILL. The doctor should definitely be noticing that, rather than reporting that "All looks good" 😆