Don't show error message when user explicitly cancel test generation

[tyuldashev]

Description

If user cancel generation at Generating test cases phase then error is shown even though cancelation was explicitly requested by user.

To Reproduce

Steps to reproduce the behavior:

1. Open utbot project
2. Start test generation for org.utbot.examples.strings.StringExamples
3. Wait till Generate test cases... phase
4. Click cross to cancel generation
   

Expected behavior

Test generation stopped. No errors shown as this is result of explicit user action.

Actual behavior

Error that no tests are generated shown with suggestion how to fix it.

Visual proofs (screenshots, logs, images)

Additional context

When cancel generation during reading classes phase then no error shown as expected