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

include/ceres/solver.h

diff --git a/include/ceres/solver.h b/include/ceres/solver.h
index 7776c47..f0d5be6 100644
--- a/include/ceres/solver.h
+++ b/include/ceres/solver.h
@@ -714,14 +714,15 @@
     // termination.
     string FullReport() const;
 
+    bool IsSolutionUsable() const;
+
     // Minimizer summary -------------------------------------------------
     MinimizerType minimizer_type;
 
-    SolverTerminationType termination_type;
+    TerminationType termination_type;
 
-    // If the solver did not run, or there was a failure, a
-    // description of the error.
-    string error;
+    // Reason why the solver terminated.
+    string message;
 
     // Cost of the problem (value of the objective function) before
     // the optimization.