Typeclass error message when a footnote is missing

[robsimmons]

Missing footnote references should lead to reasonable, Verso-document-level error messages

/--
error: failed to synthesize
  Doc.Elab.HasNote "missingfootnote" `Tests.Basic.«the canonical document object name» Doc.Genre.none

Hint: Additional diagnostic information may be available using the `set_option diagnostics true` command.
-/
#guard_msgs in
#docs (.none) errMsgForMissingFootnote "Footnote" :=
:::::::
Here's something that needs context[^missingfootnote]!
:::::::

/--
error: failed to synthesize
  Doc.Elab.HasLink "missinglink" `Tests.Basic.«the canonical document object name»

Hint: Additional diagnostic information may be available using the `set_option diagnostics true` command.
-/
#guard_msgs in
#docs (.none) errMsgForMissingLink "Footnote" :=
:::::::
Here's something that needs [context][missinglink]!
:::::::