Propagate exit code from unit tests

Also flag errors more clearly.  References #164.

Author: gaul

src/main.c

@@ -157,9 +157,7 @@ int main(int argc, char *argv[])
             break;
 
         case 't':
-            tests();
-            exit(EXIT_SUCCESS);
-            break;
+            return tests();
 
         case 'v':
             verbose = 1;

src/minunit.c

@@ -202,7 +202,7 @@ int tests()
     const char *result = all_tests();
 
     if (result != 0) {
-        printf("%s \n", result);
+        printf("Error: %s \n", result);
 
     } else {
         printf("ALL TESTS PASSED\n");