Catch & Rethrow getFieldsfromPKM

Initialization variable is captured, reset, then the pkm is loaded. If
an exception is thrown, the initialization state is not restored.
There's other capture-reset-restore cases but pkm loading is the only
one exposed to bad input.

Should prevent #261 from happening. fieldsInitialized is checked in a
bunch of events.

PKHeX/MainWindow/Main.cs

@@ -1382,7 +1382,8 @@ public void populateFields(PKM pk, bool focus = true)
                     Util.Alert("Converted File.");
             }
 
-            getFieldsfromPKM();
+            try { getFieldsfromPKM(); }
+            catch { fieldsInitialized = oldInit; throw; }
 
             CB_EncounterType.Visible = Label_EncounterType.Visible = pkm.Gen4;
             fieldsInitialized = oldInit;