Cleanup code around JERRY_UNREACHABLEs (#2342)

`JERRY_UNREACHABLE`s often signal code structure that could be
improved: they can usually either be rewritten to `JERRY_ASSERT`s
or eliminated by restructuring loops, `if`s or `#if`s. Roughly,
the only valid occurences are in default cases of `switch`es. And
even they can often be merged into non-default cases.

Moreover, it is dangerous to write meaningful code after
`JERRY_UNREACHABLE` because it pretends as if there was a way to
recover from an impossible situation.

This patch rewrites/eliminates `JERRY_UNREACHABLE`s where possible
and removes misleading code from after them.

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

jerry-core/ecma/operations/ecma-exceptions.c

@@ -82,12 +82,6 @@ ecma_new_standard_error (ecma_standard_error_t error_type) /**< native error typ
 
   switch (error_type)
   {
-    case ECMA_ERROR_COMMON:
-    {
-      prototype_id = ECMA_BUILTIN_ID_ERROR_PROTOTYPE;
-      break;
-    }
-
     case ECMA_ERROR_EVAL:
     {
       prototype_id = ECMA_BUILTIN_ID_EVAL_ERROR_PROTOTYPE;
@@ -124,9 +118,11 @@ ecma_new_standard_error (ecma_standard_error_t error_type) /**< native error typ
       break;
     }
 
-    case ECMA_ERROR_NONE:
+    default:
     {
-      JERRY_UNREACHABLE ();
+      JERRY_ASSERT (error_type == ECMA_ERROR_COMMON);
+
+      prototype_id = ECMA_BUILTIN_ID_ERROR_PROTOTYPE;
       break;
     }
   }