Assume reverted token name and symbol is mkr

[kadenzipfel]

Closes: #124

Note: This isn't an ideal fix. It basically just assumes that if a token name and symbol can't be retrieved then that token is MKR. I've never actually seen any other token that doesn't use a regular string for its name and symbol, so I don't expect we'll run into this issue again. If anyone knows of a better way to do this, please feel free.

[TomAFrench]

This could be handled by having two copies of the ERC20 ABI (one for string, one for bytes32) and if the first reverts, then second can be used.

Example of this being in the Balancer subgraph: https://github.com/balancer-labs/balancer-subgraph/blob/818d705734f4393c88abbb587dd1c6bcfad4e66d/src/mappings/helpers.ts#L84-L108