saveExecutionPlan() uses literal text in error

[Paul J. Lucas]

In xqueryimpl.cpp in saveExecutionPlan(), there is currently the line:

  ERROR_PARAMS("XML-format plan serialization", ""))

Literal text is not to be used in error messages. It must be put into the diagnostic_en.xml file.

(Assigned to Markos since, according to "bzr blame," he's the last person to touch that line.)

[Markos Zaharioudakis (markos-za)]

This code will be completely removed in 3.0, when we are allowed to make API changes.