exception: define 'internal_exception' class

internal_exception will be used for sdbusplus-generated errors.
These errors could be thrown by any sdbusplus call and therefore
by any generated server binding.  Define this class to differentiate
between binding-defined errors and sdbusplus-defined errors.

Change-Id: I725729c8e874b638cde8ce72289b87ed807d27c1
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/sdbusplus/exception.hpp b/sdbusplus/exception.hpp
index 871e85f..01687a4 100644
--- a/sdbusplus/exception.hpp
+++ b/sdbusplus/exception.hpp
@@ -8,12 +8,20 @@
 namespace exception
 {
 
+/** Base exception class for all sdbusplus exceptions, including those created
+ *  by the bindings. */
 struct exception : public std::exception
 {
 };
 
+/** base exception class for all errors generated by sdbusplus itself. */
+struct internal_exception : public exception
+{
+};
+
 } // namespace exception
 
 using exception_t = exception::exception;
+using internal_exception_t = exception::internal_exception;
 
 } // namespace sdbusplus