handle no java installed case based on exit code

jnius/jnius_jvm_dlopen.pxi

@@ -1,7 +1,7 @@
 include "config.pxi"
 import os
 from shlex import split
-from subprocess import check_output
+from subprocess import check_output, CalledProcessError
 from os.path import dirname, join, exists
 from os import readlink
 from sys import platform
@@ -60,9 +60,14 @@ cdef find_java_home():
     if platform in ('darwin'):
         # its a mac
         if not exists('/usr/libexec/java_home'):
+            # I believe this always exists, but just in case
             return
+        try:
             java = check_output('/usr/libexec/java_home').strip().decode('utf8')
             return java
+        except CalledProcessError as exc:
+            # java_home return non-zero exit code if no Javas are installed
+            return