Changes

• graph init: Skip Etherscan if --abi is provided (#263).
• graph init: Improve error when an ABI is not found on Etherscan (#266, #267).
• graph codegen: Fix ugly uncaught exception errors being printed (#262).
• graph codegen: Add notes in generated files about files being auto-generated (#212, #268).
• graph codegen: Do not generate code for calling contract functions with no return values (#270, contribution by @fubhy).
• Fail subgraph validation if callHandler functions are not found in ABIs (#265).
• Improve type suggestions for fields in the schema for integer/float types (#210, #269).
• Bump node-keytar dependency from 4.3.0 to 4.6.0 to be able to build on Node.js 12.
• Add log calls to the example to test compiling subgraphs that use logging.