Downgrade ReadOnly Exception to warning

Throwing an Exception here currently causes more pain then just
continuing.
Ideally one would rework the getter JavaProject.getResolvedClasspath()
to never modify the javamodel, but it's not clear how todo that in a
compatible way.

eclipse-jdt/eclipse.jdt.ui#1742

org.eclipse.jdt.core/model/org/eclipse/jdt/internal/core/DeltaProcessor.java

@@ -30,6 +30,7 @@
 import java.util.Set;
 import org.eclipse.core.resources.*;
 import org.eclipse.core.runtime.CoreException;
+import org.eclipse.core.runtime.ILog;
 import org.eclipse.core.runtime.IPath;
 import org.eclipse.core.runtime.IProgressMonitor;
 import org.eclipse.core.runtime.ISafeRunnable;
@@ -2022,7 +2023,9 @@ public void resetProjectCaches() {
 	 */
 	public void registerJavaModelDelta(IJavaElementDelta delta) {
 		if (JavaModelManager.isReadOnly()) {
-			throw new IllegalStateException("Its not allow to modify JavaModel during ReadOnly action. delta=" + delta); //$NON-NLS-1$
+			ILog.get().warn("JavaModel change during read only operation", new IllegalStateException( //$NON-NLS-1$
+					"JavaModel modified during 'read only' operation. Consider to report this warning to https://github.com/eclipse-jdt/eclipse.jdt.core/issues. delta=" //$NON-NLS-1$
+							+ delta));
 		}
 		this.javaModelDeltas.add(delta);
 	}