add throwable type adapter

[groov1kk]

Hey!

When trying to run tests from an IDE using the JUnit runner in a hybrid Kotlin/Java project (for example, in case if a project uses this template) I get errors like:

java.lang.IllegalArgumentException: Unable to create converter for class com.intellij.remoterobot.client.RetrieveResponse
    for method IdeRobotApi.screenshot

Caused by: com.google.gson.JsonIOException: Failed making field 'java.lang.Throwable#detailMessage' accessible; either increase its visibility or write a custom TypeAdapter for its declaring type.

This happens because of the difference between java.lang.Throwable and kotlin.Throwable which Robot framework is using in his API model, as the Java version of Throwable type also contains hidden members. Actually, Gson does not contain ThrowableTypeAdapter out of the box and does not know how to deserialize this type, as it was mention here.

So, adding a ThrowableTypeAdapter explicetly may solve this issue.