imp: use error returns in VerifyMembership fns

[colin-axner]

Description

made this change in relation to cosmos/ics23#391

Example error message now:

failed to verify membership proof at index 0: provided value doesn't match proof: invalid proof

Previous error message:

chained membership proof failed to verify membership of value: 696E76616C69642076616C7565 in subroot FED126FA5980E8E5261C96B693BB0DB1999AA07C23CD5A53F23D59340A3F4A1C at index 0. Please ensure the path and value are both correct.: invalid proof

closes: #XXXX

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[colin-axner]

decided to take the liberty to do some code cleanup. Since proof.Proofs.GetNonexist only returns a non exist proof, the switch statement is unnecessary since we check the return value is non nil

[colin-axner]

ditto