Consolidate SolverTerminationType enum.

1. Rename SolverTerminationType to TerminationType.
2. Consolidate the enum as
  a. CONVERGENCE - subsumes FUNCTION_TOLERANCE, PARAMETER_TOLERANCE and GRADIENT_TOLERANCE
  b. NO_CONVERGENCE
  c. FAILURE - captures all kinds of failures including DID_NOT_RUN.
  d. USER_SUCCESS
  e. USER_FAILURE
3. Solver::Summary::error is renamed to be Solver::Summary::message, to both
reduce confusion as well as capture its true meaning.

Change-Id: I27a382e66e67f5a4750d0ee914d941f6b53c326d
diff --git a/internal/ceres/minimizer.cc b/internal/ceres/minimizer.cc
index 2e2c15a..bdb6a11 100644
--- a/internal/ceres/minimizer.cc
+++ b/internal/ceres/minimizer.cc
@@ -54,7 +54,7 @@
       VLOG(1) << "Terminating: User callback returned USER_SUCCESS.";
       return false;
     case SOLVER_ABORT:
-      summary->termination_type = USER_ABORT;
+      summary->termination_type = USER_FAILURE;
       VLOG(1) << "Terminating: User callback returned  USER_ABORT.";
       return false;
     default: