build: fix incorrect error handling in ZiglingStep.compile

When handling the error from the eval method, some possible errors are
ignored.  The make method will only print the exercise hint and the
help message.

Print the unexpected error message, in the else prong.
Note that FileNotFound can also be considered unexpected.

build.zig

@@ -395,7 +395,16 @@ const ZiglingStep = struct {
                     for (argv) |v| print("{s} ", .{v});
                     print("\n", .{});
                 },
-                else => {},
+                else => {
+                    print("{s}{s}: Unexpected error: {s}{s}\n", .{
+                        red_text,
+                        self.exercise.main_file,
+                        @errorName(err),
+                        reset_text,
+                    });
+                    for (argv) |v| print("{s} ", .{v});
+                    print("\n", .{});
+                },
             }
 
             return err;