Add .DELETE_ON_ERROR to Makefile

This instructs make to delete target files if the recipe fails midway, like
when make itself is interrupted. This is mostly for development since it
protects against corrupt builds that need a `make clean` to repair. Release
builds normally don't fail mid-recipe.

GNU make and BSD make support this.

From GNU make's manual:

> This is almost always what you want make to do, but it is not historical
> practice; so for compatibility, you must explicitly request it. 

See https://innolitics.com/articles/make-delete-on-error/

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