Mock server: catch errors in graphql execution

[MetRonnie]

E.g. doing this named query without the $id arg:

query LogFiles {
  logFiles (id: "foo") {
    files
  }
}

would crash the nodemon server. Now it returns the proper error

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Tests, changelog, docs not needed