Crashes in QueryParser

[shuebner]

There are some errors in the QueryParser that make it throw unexpected exceptions:

1. ParseFieldsQuery throws NullReferenceException when the field in the query doesn't exist, because of unchecked SingleOrDefault() (same thing in ContextGraph.GetRelationshipName btw)

2. Page query parsing crashes with some malformed queries, e. g. ParsePageQuery will crash when querying with ?page[size]=a. This should probably be a TryParse instead of a ToInt32

I can prepare a PR if you want.