rest_vol_dataset: (fix) is_contiguous() and offset

- Improved the function RV_dataspace_selection_is_contiguous() and
  RV_convert_start_to_offset at suggested in pull request HDFGroup#58
  on Github. In short, now the switch statements handle the various
  dataspace types correctly.

src/rest_vol_dataset.c

@@ -4302,11 +4302,17 @@ RV_dataspace_selection_is_contiguous(hid_t space_id)
             break;
 
         case H5S_SEL_ALL:
+            FUNC_GOTO_DONE(TRUE);
+            break;
+
+        case H5S_SEL_NONE:
+            FUNC_GOTO_DONE(FALSE);
+            break;
+
         case H5S_SEL_ERROR:
         case H5S_SEL_N:
-        case H5S_SEL_NONE:
         default:
-            FUNC_GOTO_DONE(TRUE);
+            FUNC_GOTO_ERROR(H5E_DATASPACE, H5E_BADVALUE, FAIL, "specified unsupported dataspace type");
     } /* end switch */
 
 done:
@@ -4379,11 +4385,14 @@ RV_convert_start_to_offset(hid_t space_id)
             break;
 
         case H5S_SEL_ALL:
+        case H5S_SEL_NONE:
+            ret_value = 0;
+            break;
+
         case H5S_SEL_ERROR:
         case H5S_SEL_N:
-        case H5S_SEL_NONE:
         default:
-            ret_value = 0;
+            FUNC_GOTO_ERROR(H5E_DATASPACE, H5E_BADVALUE, -1, "specified unsupported dataspace type");
     } /* end switch */
 
 done:
@@ -4503,4 +4512,4 @@ RV_json_values_to_binary_callback(char *HTTP_response, void *callback_data_in, v
         RV_free(value_buffer);
 
     return ret_value;
-}
+}