Clearer errors when decoding to invalid types

[arp242]

When passing an invalid type such as "int" you would get the rather
unhelpful message of:

toml: cannot load TOML value of type map[string]interface {} into a Go integer

This adds an extra check for that, so that you get:

toml: cannot decode to type int

The second case is when you're using a wrong map type or struct field
type, the previous error message was:

toml: cannot load TOML value of type bool into a Go integer

So what exactly failed? No clue at all. This adds the TOML key:

toml: incompatible types: TOML key "F" has type bool; destination has type integer

Also clarify the errors for non-pointers and nil a wee bit while I'm
here.

Fixes #158