Error handling of WithdrawValidatorCommission in export.go

[jaybxyz]

Summary

While I was implementing TestAppSimulationAfterImport simulation test for the fundraising module, I encountered this panic panic: no validator commission to withdraw. This is due to the error handling in WithdrawValidatorCommission function in app/export.go. I think it is worth of sharing with the SPN team to minimize debugging time if there is a plan to implement the simulation test.

To share a little more in detail, the simulation of Cosmos SDK staking module allows to set a validator's commission rate as zero. See this lines of code for reference. This may be the reason why error handling is ignored in CanWithdrawInvariant and here.

[lumtis]

Thanks for the report, I think we haven't implemented this type of simulation test so far