Reading metadata from a non-WebAssembly file creates an infinite iterator

[Michael-F-Bryan]

Because of the way wasmparser's Parser::parse_all() method is implemented, triggering a "bad magic number" error won't let the parser make any progress, meaning if you don't break on errors you'll get an infinite iterator.

Later on we skip any errors and try to iterate over every Payload in the infinite iterator, causing commands like rune inspect and rune run to run forever.

Fixes #315.