DATACMNS-380 - MappingContext exposes PersistentPropertyPath by dot path.

Introduced MappingContext.getPersistentPropertyPath(String, Class<?>) to allow looking up a PersistentPropertyPath from a plain dot-notated path expression (e.g. foo.bar.foobar). This removes the need to use a PropertyPath in case you really only want to look up plain path expressions and the additional functionality in PropertyPath (camel case traversal and interpreting _ as hard delimiter) is not needed or wanted.
DATACMNS-381 - Improved exception message in PropertyReferenceException.

PropertyReferenceException now not only exposes the property not found but also the already resolved Property path to make it easier to understand what is causing the exception in the first place.