misc/wasm: improve error message if javascript polyfills are required

wasm_exec.js expects that either "require" is available or that the
globals "crypto", "TextEncoder" and "TextDecoder" are already defined.
Report a better error message if this is not the case, suggesting the
use of a polyfill.

Updates #41482

Change-Id: I5473cae15c98ae42e39f5928245b7762e7a5a8bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/261357


Trust: Richard Musiol <neelance@gmail.com>
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>