Add unit tests for lastError

Author: d-torrance

Diff for: M2/Macaulay2/tests/normal/error-messages.m2

@@ -1,3 +1,7 @@
+assert(try 1/0 else lastError#1 == "division by zero")
+lastError = null
+assert(lastError === null)
+
 stderr << "--testing the error messages must be done manually" << endl
 end