zeroing: Add API to flag errors.

The zeroing class itself won't poke any other processes or classes.
Whatever code is using the zeroing class needs to poll for an error
state using the `error()' function.

Change-Id: Ie872d923c6c85419e4ca55680ab6c7437e284c89
diff --git a/frc971/zeroing/zeroing.cc b/frc971/zeroing/zeroing.cc
index 92f2875..d45c49f 100644
--- a/frc971/zeroing/zeroing.cc
+++ b/frc971/zeroing/zeroing.cc
@@ -24,6 +24,14 @@
   zeroed_ = false;
   wait_for_index_pulse_ = true;
   last_used_index_pulse_count_ = 0;
+  error_ = false;
+}
+
+void ZeroingEstimator::TriggerError() {
+  if (!error_) {
+    LOG(ERROR, "Manually triggered zeroing error.\n");
+    error_ = true;
+  }
 }
 
 double ZeroingEstimator::CalculateStartPosition(double start_average,