change "missing up reference" to concrete level name

[florianschanda]

Example 1:

requirements "Requirements" {
   source: "trlc.lobster";
}

implementation "Code" {
	trace to: "Requirements";
       source: "python.lobster";
}

If an item from python.lobster does not contain a link to an item from trlc.lobster, then currently the HTML report displays the message "missing up reference".
Instead, it shall print "missing up reference to Requirements".

Example 2:

requirements "Level A1" {
   source: "a1.lobster";
   requires: "Code";
}

requirements "Level A2" {
   source: "a2.lobster";
   requires: "Code";
}

implementation "B" {
	trace to: "A1";
        trace to: "A2";
       source: "b.lobster";
}

If an item from b.lobster does not contain a link to an item from a1.lobster, and also no link to an item from a2.lobster, then the HTML report shall print the following messages:

1. "missing up reference to A1"
2. "missing up reference to A2"

Once #10 is implemented, the messages must be more sophisticated. They must include all optional sources.