[manifest] Icon density parsing should have a developer warning (#282)

```If result is NaN, +∞, or less than or equal to +0, let result be 1.0.```
Should probably be:
```
If result is NaN, +∞, or less than or equal to +0:
 - issue a developer warning [...].
 - let result be 1.0.
```

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/issues/282