This action should report broken references

[fingolfin]

Looking at e.g. this log I see messages like this:

#I Second run through document . . .
#W WARNING: non resolved reference: rec(
  BookName := "ref",
  Oper := "ViewObj" )
#W WARNING: non resolved reference: rec(
  BookName := "ref",
  Oper := "PrintObj" )

The cause of this is discussed in issue #17, but what I want to focus on here is that the CI job still is reported as successful, and only by drilling into the CI logs does one notice this problem.

I think that at the very least, developers should be notified about this. Perhaps by making the build fail. Or perhaps by showing a "warning message" in the CI -- or perhaps both?

For this we need a way to extract these warnings from the output. Then it is easy to generate GitHub "warning messages" (see here for an explanation what this) which in turn make it easy to get to the exact spot in the logs.