Fuzzing: Handle instantiation errors in ClusterFuzz

[kripken]

If the module has an invalid segment offset, for example, it will error. We
should not error in the fuzzer on such rare cases.

Also add logging of the actual error, which matches what we do for
errors elsewhere, and makes debugging easier:

binaryen/scripts/fuzz_shell.js

Line 397 in dcec348

console.log('exception thrown: ' + e);

As a result another place needs to look for the prefix now, and not the
entire string (since we append the error contents).

[kripken]

friendly new year's ping @tlively (no rush)