Not possible to sign_message on tx_service mode on thresholds bigger than 1

[JagoFigueroa]

Describe the bug
We have an issue with sign_message where after loading 2 owners (via private keys, not hardware wallets) and trying to sign a message it will throw an error.

To Reproduce
Load a safe with 2 owners and execute sign_message command.

(Example flow with info of the loaded safe that is throwing the error in my case)

tx-service > 0x4E24f2497445125C4573f7935ADeDEce154317b8 > info
Address=0x4E24f2497445125C4573f7935ADeDEce154317b8
Nonce=33
Threshold=2
Owners=['0x35E730d5Ce84F9C0D7F48F15337e80341B1d621c', '0x72d315603Ba7eE9C100f4207a80248F58381A20d', '0x2986Be7eaE053d47670806277a5a70Ed859bCf90', '0xfca7Da0a0290D7BcBEcD93bE124756fC9B6F8E6A', '0xc6b82bA149CFA113f8f48d5E3b1F78e933e16DfD']
Master_copy=0x34CfAC646f301356fAa8B21e94227e3583Fe3F5F
Modules=[]
Fallback_handler=0xd5D82B6aDDc9027B22dCA772Aa68D5d74cdBdF44
Guard=0x0000000000000000000000000000000000000000
Balance_ether=0.06
Version=1.1.1
Safe Tx Service=https://safe-transaction-gnosis-chain.safe.global/api/v1/safes/0x4E24f2497445125C4573f7935ADeDEce154317b8/transactions/
Etherscan=https://gnosisscan.io/address/0x4E24f2497445125C4573f7935ADeDEce154317b8
Ledger=supported
Trezor=supported
Safe is not updated! You can use update command to update the Safe to a newest version
tx-service > 0x4E24f2497445125C4573f7935ADeDEce154317b8 > show_cli_owners
Account 0x35E730d5Ce84F9C0D7F48F15337e80341B1d621c loaded
Account 0xc6b82bA149CFA113f8f48d5E3b1F78e933e16DfD loaded
Default sender: 0xc6b82bA149CFA113f8f48d5E3b1F78e933e16DfD
tx-service > 0x4E24f2497445125C4573f7935ADeDEce154317b8 > sign_message --eip191_message I_love_Doritos
Error posting message: b'{"nonFieldErrors":["1 owner signature was expected, 2 received"]}'

[moisses89]

After talk with @Uxio0 it is expected that the safe-transaction-service doesn't let create messages with more than one signer.
Closing this issue in favor of #346