Tweak visualizer interface

Report visualization generation errors through an explicit
IApplication interface, instead of pulling messages out of the
DebugLog stream.

Declare that GetVisGenDescrs() is only called when the plugin is in
the "prepared" state, so that plugins can taylor the set based on
the contents of the file.  (This could be used to set min/max on
the "offset" entries, but I want special handling for offsets, so
we might as well set it later.)

SourceGen/CodeAnalysis.cs

@@ -76,6 +76,10 @@ namespace SourceGen {
                 mOuter = null;
             }
 
+            public void ReportError(string msg) {
+                DebugLog(msg);
+            }
+
             public void DebugLog(string msg) {
                 mOuter.mDebugLog.LogI("PLUGIN: " + msg);
             }