[WIP] Replace throws with invariants where appropriate

Also:
* adds a new expr class
* fixes regression test that doesn't work with newer/stricter perl
* changes return code on one regression test because its changed by the PR
  (If that's unwanted I can change it back though)

Marked as WIP because we'll likely want to replace io_exception and
interpreter_exception with system_exception and analyses_exception

Author: hannes-steffenhagen-diffblue

regression/cbmc/return6/test.desc

@@ -1,7 +1,7 @@
 CORE
 main.c
 f_def.c
-^EXIT=6$
+^EXIT=1$
 ^SIGNAL=0$
 CONVERSION ERROR
 --

src/goto-programs/interpreter_class.h

@@ -23,6 +23,7 @@ Author: Daniel Kroening, [email protected]
 #include "goto_functions.h"
 #include "goto_trace.h"
 #include "json_goto_trace.h"
+#include "interpreter.h"
 
 class interpretert:public messaget
 {
@@ -279,8 +280,9 @@ class interpretert:public messaget
   {
     mp_vectort v;
     evaluate(expr, v);
+    // FIXME not sure if this can happen
     if(v.size()!=1)
-      throw "invalid boolean value";
+      throw interpreter_errort("invalid boolean value");
     return v.front()!=0;
   }