observatory herald (#30)

* WIP: initial commit for observatory herald

* Plugin error handling refactor

* make error window non-modal

* tidy up plugin error handling

* first pass for basic herald functionality

* corrections for linux env

* Use FNV hash directly instead of managing through dictionary/index file

* resolve audio queuing issue, switch to personal NetCoreAudio fork

* merge cleanup

* add enable setting, populate defaults

* framework xml doc update

* Adjust settings, add style selection, replace locale with demonym in dropdown list.

* Test is position is on screen before saving/loading.

* use a default that's actually in the list

ObservatoryFramework/Exceptions.cs

@@ -0,0 +1,34 @@
+using System;
+
+namespace Observatory.Framework
+{
+    /// <summary>
+    /// Container for exceptions within plugins which cannot be gracefully handled in context,
+    /// but benefit from having a context-specific user message.
+    /// </summary>
+    public class PluginException : Exception
+    {
+        /// <summary>
+        /// Initialze new PluginException with details of the originating plugin and a specific user-facing message for display.
+        /// </summary>
+        /// <param name="pluginName"></param>
+        /// <param name="userMessage"></param>
+        /// <param name="innerException"></param>
+        public PluginException(string pluginName, string userMessage, Exception innerException) : base(innerException.Message, innerException)
+        {
+            PluginName = pluginName;
+            UserMessage = userMessage;
+        }
+
+        /// <summary>
+        /// Name of plugin from which the exception was thrown.
+        /// </summary>
+        public string PluginName { get; }
+
+        /// <summary>
+        /// Message to be displayed to user.
+        /// </summary>
+        public string UserMessage { get; }
+        
+    }
+}