Jaeger: Improve JSON file upload error msg

public/app/plugins/datasource/jaeger/datasource.test.ts

@@ -113,7 +113,7 @@ describe('JaegerDatasource', () => {
         targets: [{ queryType: 'upload', refId: 'A' }],
       } as any)
     );
-    expect(response.error?.message).toBeDefined();
+    expect(response.error?.message).toBe('The JSON file uploaded is not in a valid Jaeger format');
     expect(response.data.length).toBe(0);
   });
 

public/app/plugins/datasource/jaeger/datasource.ts

@@ -89,7 +89,7 @@ export class JaegerDatasource extends DataSourceApi<JaegerQuery, JaegerJsonData>
         }
         return of({ data });
       } catch (error) {
-        return of({ error: { message: 'JSON is not valid Jaeger format' }, data: [] });
+        return of({ error: { message: 'The JSON file uploaded is not in a valid Jaeger format' }, data: [] });
       }
     }