Sparta Assert for Target Pipe without Execution Unit (#151)

Forgot to add an assert to check that there is a dispatcher (thus a
subsequent issue queue/execution unit) that can handle a target pipe of
an instruction. If one forgets to define one, i.e you forget to define
an execution unit to handle `DIV` target pipe, the simulator will end
early, but no error will be thrown.

core/Dispatch.cpp

@@ -239,6 +239,11 @@ namespace olympia
                                               << " not found in dispatchers, make sure pipe is "
                                                  "defined and has an assigned dispatcher");
                 auto & dispatchers = dispatchers_[static_cast<size_t>(target_pipe)];
+                sparta_assert(dispatchers.size() > 0,
+                              "Pipe Target: "
+                                  << target_pipe
+                                  << " doesn't have any execution unit that can handle that target "
+                                     "pipe. Did you define it in the yaml properly?");
                 // so we have a map here that checks for which valid dispatchers for that
                 // instruction target pipe map needs to be: "int": [exe0, exe1, exe2]
                 if (target_pipe != InstArchInfo::TargetPipe::LSU)