imagej.doctor should notice subprocess crashes

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 .

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 .

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" :laughing:

Start at the imagej.doctor checkup and inspect how its Maven and Java subprocess calls report failures. Reproduce the SIGKILL behavior with the doctor command, then verify that a crashed subprocess is reported as a failed check instead of producing “All looks good.”

Written by the indexing model from the issue text.