#163 nits: refuse LD_*/DYLD_* in process:env, document receipt grant literals, don't leak runtime-error JSON to HTTP clients #166

From Motoko's #163 re-review: (1) process:env= accepts LD_PRELOAD, LD_, DYLD_ — refuse or document. (2) Document that receipt grant strings copy pattern literals verbatim. (3) Handler errors return full runtime-error JSON to the HTTP client (predates #163).