Add a load result interface for use in new load_rules methods

Define a falco_load_result abstract class for use in new load_rules
methods. It's abstract so the implementation details in
rule_loader/rule_reader can be hidden from someone who wants to use
the API to load rules and work with a result.

The class defines a set of error codes/warning codes and has static
methods to get a short and long description of each error/warning.

There are virtual methods to access the important parts of a result:
 - successful or not
 - a string representation of the result, suitable for display to
   users. Takes a verbose argument. When verbose is true, the string is
   multi-line and has full details, including locations, item names,
   etc. When verbose is false, the string is single-line and just
   returns error codes.
 - a json representation of the result, suitable for automated
   parsing/interpretation later.

Signed-off-by: Mark Stemm <mark.stemm@gmail.com>

userspace/engine/CMakeLists.txt

@@ -13,6 +13,7 @@
 set(FALCO_ENGINE_SOURCE_FILES
     falco_common.cpp
     falco_engine.cpp
+    falco_load_result.cpp
     falco_utils.cpp
     json_evt.cpp
     evttype_index_ruleset.cpp