Kernel+LibELF: Abort ELF executable load sooner when something fails

Make it possible to bail out of ELF::Image::for_each_program_header()
and then do exactly that if something goes wrong during executable
loading in the kernel.

Also make the errors we return slightly more nuanced than just ENOEXEC.

DevTools/UserspaceEmulator/Emulator.cpp

@@ -208,8 +208,10 @@ bool Emulator::load_elf()
                 m_loader_text_size = region->size();
             }
             mmu().add_region(move(region));
-            return;
+            return IterationDecision::Continue;
         }
+
+        return IterationDecision::Continue;
     });
 
     auto entry_point = interpreter_image.entry().offset(interpreter_load_offset).get();