Issue a nicer error message on wasm components.

[sunfishcode]

Decode just enough of the component binary format to recognize when the input is a component, and issue a dedicated error message for it.

Before:

0000008: error: bad wasm file version: 0x1000d (expected 0x1)

After:

0000008: error: wasm components are not yet supported in this tool

[SoniEx2]

this should probably have a dedicated test for it fwiw