Readme update, to contain many more examples.

Change the behavior of serum.Message to fallback to Error string on n…

…on-serum types.

This seems to produce slightly less surprising behavior in the wild;
and it means less information is discarded, which in any scenario that
might contain a flicker of doubt, is generally the right way to go.

This also means that when Standardize functions are used (or, the
Errorf helper constructor, which uses Standardize in turn) on non-serum
error values, they keep their messages better.  Big improvement.

Add ToJSONString helper.

It's very trivial, but it's also very very very useful to get
the returns down to a single value, for printf composition purposes.

Code should return emptystring calmly for nil errors.

This improves ease of use.

introduce Error constructor, with optional style varargs construction…

…, and an ability to access the template system.