introduced ReadOnlyError to allow differentiated error handling

the immediate purpose is communicating to tiddlywebplugins.imaker that
it should not create entities in these stores - the assumption is that
when this error is raised, diststore is being used in conjunction with
pkgstore to read contents from the package at runtime rather than
maintaining a copy within the respective instance
Merge pull request #1 from FND/readonly

introduced ReadOnlyError to allow differentiated error handling