Cosmetic change: reorder imports
Change error methods to accept variable arguments rather than a string

Also, add f variants to the shorthand methods. This allows more
flexibility as well as calling the shorthand functions with no
arguments, which will show the default message for the given code.
Remove Context.SetCached and Context.SetServedFromCache

Use Context.Set and strings defined in gnd.la/internal to provide
the same functionality without exporting a couple of methods which
users are not supposed to call.
When making assets, don't try to compile files starting with .
Panic when the Context functions for parsing a value don't get a ptr

This allows the user to easily see they made a programming error.
Datastore: Make all transactions cross-group

While this is not the optimal solution when it comes to performance,
it's a valid one. If later we find this is a signficant bottleneck,
we will provide an specific option.
Datastore: avoid creating nested transactions

Some driver functions need to run in a transacion, but the caller
might have already set up one for us, so we need to take that
into account because the datastore does not support nesting
transactions.