Better error reporting

[kalekundert]

Implement Passport/Breadcrumb/Log system to let Getters (and maybe Layers/Configs) annotate what they're doing.

[kalekundert]

Some examples of specific problems, to help with architecting:

• freezerbox.ReagentConfig: the database is loaded in __getitem__(), which seems to be invoked after get_location(). As a result, the error traces report "no database found" when this is not the case.
• freezerbox.ReagentConfig: accidentally forgot to properly define tag_getter; error message just says "can't find value for parameter", doesn't suggest that tag_getter is the problem.

[kalekundert]

I think this will require making Layer a more active object. Some pseudocode:

• model.iter_values() begins by instantiating an empty log.
• The log is passed to each getter via getter.iter_values().
  • Method/Func/Value directly log something like "calling {self.func.__name__}: returned {value}/raised {exc}"
  • Key would pass the log on to each layer it encounters. Maybe each config too, e.g. to log if the config is loaded or not.
• Layer could be subclassed:
  • The standard layer would require a location, and include it in the log messages.
  • The FreezerBox configs might use a custom Layer, which would include information about the database rather than requiring an explicit location.

[kalekundert]

Schema parsing should also be delegated to the Layer class, see #16.