Fix TypeScript errors

[EricImpact]

@ts-expect-error directive is not needed; nothing wrong with defining tus as undefined.

Do some type narrowing of err in catch block to confirm it's an Error before returning message.

Closes #5592

[github-actions]

Diff output files

diff --git a/packages/@uppy/core/lib/Uppy.js b/packages/@uppy/core/lib/Uppy.js
index b4c89fe..05a12aa 100644
--- a/packages/@uppy/core/lib/Uppy.js
+++ b/packages/@uppy/core/lib/Uppy.js
@@ -520,7 +520,9 @@ export class Uppy {
     try {
       _classPrivateFieldLooseBase(this, _restricter)[_restricter].validateSingleFile(file);
     } catch (err) {
-      return err.message;
+      if (err instanceof Error) {
+        return err.message;
+      }
     }
     return null;
   }
@@ -529,7 +531,9 @@ export class Uppy {
     try {
       _classPrivateFieldLooseBase(this, _restricter)[_restricter].validateAggregateRestrictions(existingFiles, files);
     } catch (err) {
-      return err.message;
+      if (err instanceof Error) {
+        return err.message;
+      }
     }
     return null;
   }