makefile.tmpl: fix error handling and cleanups
makefile.tmpl: fix error handling and cleanups

This changes the indentation of all non-recipe lines from tabs to 2
spaces. While this doesn't fix anything, it avoids either make or
someone interpreting those lines as a make recipe.

This also fixes the error handling by using make's $(error) function
instead. Previously make gave this syntax error whenever it hit the
error handling code:

```
*** recipe commences before first target.  Stop.
```

russpowers/Nim,russpowers/Nim,dom96/Nim,tulayang/Nim,russpowers/Nim,dom96/Nim,tulayang/Nim,tulayang/Nim,dom96/Nim,dom96/Nim,tulayang/Nim,dom96/Nim,russpowers/Nim,russpowers/Nim,tulayang/Nim,tulayang/Nim,russpowers/Nim,russpowers/Nim,dom96/Nim,dom96/Nim