Support better failure descriptions in DocumentsMatchConstraint

DocumentsMatchConstraint currently makes it difficult to figure out where the comparison failure occurred when checking embedded documents. A few things can be done to improve things:

• Using $this->exporter to stringify the expected document
• Throwing a ComparisonFailure instead of a RuntimeException would allow for seeing a diff of the compared documents
• Using additionalFailureDescription allows for adding more information as to what failed.