Add error output when ABI is not generated using "eosio-cpp"

[bogniq]

Change Description

When using eosio-cpp to generate WASM and ABI, if the contract is empty, or if the specified contract name doesn't match the real contract name, the ABI will silently not be generated. This fix adds warning output, or throws exception with error messages, for this situation.

Notes:

• Only when --abigen option is set, the warning or error is output. This is to avoid incompatibility with some previous code (e.g. some contracts in the unit test code have different names for contract and filename). This decision can be further discussed.
• The specified contract name is either explicitly specified by --contract, or inferred from output name by -o, or the same as the contract filename if neither '--contract' or '-o' is set.

API Changes

• API Changes

Documentation Additions

• Documentation Additions