Throw a WoodmanError instead of a mere Error when load fails (#15)

When configuration is invalid, Woodman now throws (or reports when
a callback function is provided) a WoodmanError that can be used e.g.
to ensure a server stops when traces cannot be initialized.

The error class is exposed to the external world as "woodman.error".

The precompiler still needs to be updated to replace references to
"woodman.error" with "function () {}" in lines such as:
 if (err instanceof woodman.error) { ... }
Renamed "woodman.error" into "woodman.Error" (#15)
Precompiler: added support for unknown logger and woodman functions

The precompiler now supports removal of unknown trace functions that
may have been registered with calls to "registerLevel".

It also supports removing calls to "registerAppender", "registerLevel"
and so on and replaces references to "woodman.Error" with
"function () {}" in code such as:
 if (err instanceof woodman.Error)