Exception when serializing error model library which extends error model library

[philip-alldredge]

Expected and Current Behavior

When saving an AADL resource which contains an error mode library which extends another one when the text editor isn't open, an exception is thrown and logged. This prevents any changes to the error model library from being saved.

Steps to Reproduce

1. Open a package diagram for a package which has an error model library extension. other in the example below.
2. Make any change to the model using the diagram editor when the text editor isn't open.
3. Exception is thrown. If the change was an error model library change, the model is not updated

A stack trace is attached but it does not uncover the root cause. The issue appears to be retrieving the IContext fails. Ultimately ErrorModelCrossReferenceSerializer.getCrossReferenceNameFromScop is called and returns null. At that point things bubble up to ultimately not find the context. I suspect that serializing this function may be the solution.

package other
public

	annex EMV2 {**
		error types
	extends emv2_test with
		end types;
	**};
end other;

package emv2_test
public

	annex EMV2 {**
		error types
		end types;
	**};
end emv2_test;

This is a blocker for #2450