Change errors switch example into zig test output (#443)

This example was being built via zig build-exe but is written as a
zig test example. The output on the website is an error about not
having a main function instead of an error about not specifying
all possibilities of an error in a switch.

Author: josephmr

assets/zig-code/features/15-errors-switch.zig

@@ -32,4 +32,4 @@ fn charToDigit(c: u8) !u8 {
     return value;
 }
 
-// exe=build_fail
+// test_safety=possibilities