png2src return value does not report errors to the caller
If png2src encounters an error, it does not report it via its return value:

```bash
$ w4 png2src --zig lib/test/data/smile_8x8_5colors.png
Error processing lib/test/data/smile_8x8_5colors.png: 
Too many colors: maximum is 4. The previous colors were:
- (R: 0, G: 0, B: 0, A: 255); first seen at (0, 0)
- (R: 255, G: 255, B: 255, A: 255); first seen at (2, 0)
- (R: 0, G: 255, B: 0, A: 255); first seen at (2, 2)
- (R: 255, G: 0, B: 0, A: 255); first seen at (2, 5)
The first occurrence of another color is at (3, 6) and has the value of (R: 0, G: 0, B: 255, A: 255)
```

Expected behavior:
```bash
$ echo $?
1
```

Actual behavior:
```bash
$ echo $?
0
```
