Fixes for some line search bugs & corner cases.

- Increase precision of numeric values output in error messages to
  allow for easier debugging.
- Ensure termination after Wolfe search bracketing phase if bracket
  width has been shrunk to below tolerance.
- Cleaned up return value for BracketingPhase(), now false iff
  optimisation should stop, true otherwise.
- Fix bug whereby we would mark a step size as satisfying the Wolfe
  conditions when it did not due to numerical issues in the cost
  function.
- Adding explanation of a subtlety in which a zoom could still be
  acceptably invoked with bracket_low.f > bracket_high.f.
- Replacing hard check of a pre-condition of ZoomPhase() with a
  conditional return if not satisfied to address issue whereby a
  bracket could be incorrectly identified due to inconsistent values
  & gradients returned from the cost function.
- Adding missing check for step size validity in line search minimizer.
- Adding ToDebugString() for FunctionSample.

Change-Id: Iad98e635749877f80c079ebad126bf022d82232d

internal/ceres/polynomial.h

@@ -95,6 +95,7 @@ struct FunctionSample {
         gradient(0.0),
         gradient_is_valid(false) {
   }
+  string ToDebugString() const;
 
   double x;
   double value;      // value = f(x)