This converts the majority of script errors from generic errors created
via errors.New and fmt.Errorf to use a concrete type that implements the
error interface with an error code and description.

This allows callers to programmatically detect the type of error via
type assertions and an error code while still allowing the errors to
provide more context.

For example, instead of just having an error the reads "disabled opcode"
as would happen prior to these changes when a disabled opcode is
encountered, the error will now read "attempt to execute disabled opcode
OP_FOO".

While it was previously possible to programmatically detect many errors
due to them being exported, they provided no additional context and
there were also various instances that were just returning errors
created on the spot which callers could not reliably detect without
resorting to looking at the actual error message, which is nearly always
bad practice.

Also, while here, export the MaxStackSize and MaxScriptSize constants
since they can be useful for consumers of the package and perform some
minor cleanup of some of the tests.