Add defcustom to control default error level for Flymake.

This creates a custom setting similar to what already exists for
Flycheck.  Additionally, the default value is now made consistent with
the default value for Flycheck (i.e., error instead of note).

lsp-diagnostics.el

@@ -58,6 +58,15 @@
           (const info))
   :group 'lsp-diagnostics)
 
+(defcustom lsp-diagnostics-flymake-default-level :error
+  "Error level to use when the server does not report back a diagnostic level."
+  :type '(choice
+          (const :tag "Error"   :error)
+          (const :tag "Warning" :warning)
+          (const :tag "Note"    :note))
+  :group 'lsp-diagnostics
+  :package-version '(lsp-mode . "9.0.1"))
+
 (defcustom lsp-diagnostics-attributes
   `((unnecessary :foreground "gray")
     (deprecated  :strike-through t))
@@ -311,7 +320,9 @@ See https://github.com/emacs-lsp/lsp-mode."
                                                (cl-case severity?
                                                  (1 :error)
                                                  (2 :warning)
-                                                 (t :note))
+                                                 (3 :note)
+                                                 (4 :note)
+                                                 (t lsp-diagnostics-flymake-default-level))
                                                message))))
            ;; This :region keyword forces flymake to delete old diagnostics in
            ;; case the buffer hasn't changed since the last call to the report