imagej doctor should recognize non-zero return values

[hinerm]

From this failure on windows:

Checking Maven:
--> Maven executable = C:\Users\chiarutt\AppData\Local\abba-python\Library\bin\mvn.CMD
$ mvn -v
Command '['C:\\Users\\chiarutt\\AppData\\Local\\abba-python\\Library\\bin\\mvn.CMD', '-v']' returned non-zero exit status 1.

...

Great job! All looks good.