improve mixpanel usage statistics (#2238)

Other changes:
- Always display first start dialog if privacy options are unset (e.g. if the user closed GPT4All without selecting them)
- LocalDocs scanQueue is now always deferred
- Fix a potential crash in magic_match
- LocalDocs indexing is now started after the first start dialog is dismissed so usage stats are included

Signed-off-by: Jared Van Bortel <jared@nomic.ai>

gpt4all-backend/llmodel.h

@@ -17,6 +17,29 @@ class LLModel {
 public:
     using Token = int32_t;
 
+    class BadArchError: public std::runtime_error {
+    public:
+        BadArchError(std::string arch)
+            : runtime_error("Unsupported model architecture: " + arch)
+            , m_arch(std::move(arch))
+            {}
+
+        const std::string &arch() const noexcept { return m_arch; }
+
+    private:
+        std::string m_arch;
+    };
+
+    class MissingImplementationError: public std::runtime_error {
+    public:
+        using std::runtime_error::runtime_error;
+    };
+
+    class UnsupportedModelError: public std::runtime_error {
+    public:
+        using std::runtime_error::runtime_error;
+    };
+
     struct GPUDevice {
         int index;
         int type;
@@ -53,7 +76,8 @@ public:
         static const Implementation *implementation(const char *fname, const std::string &buildVariant);
         static LLModel *constructDefaultLlama();
 
-        bool (*m_magicMatch)(const char *fname);
+        char *(*m_getFileArch)(const char *fname);
+        bool (*m_isArchSupported)(const char *arch);
         LLModel *(*m_construct)();
 
         std::string_view m_modelType;