sandstorm/sandstorm-http-bridge.c++: If execvp() fails, tell user they might need to 'chmod a+x'

[paulproteus]

Rationale: https://botbot.me/freenode/sandstorm/2016-02-24/?msg=60803986&page=1

[kentonv]

It should be easily possible to detect if executable permissions are the problem (vs. the file not existing at all) with some calls to access(). Please only print a specific suggestion if it is the correct one, otherwise it's really confusing when the suggestion is wrong.

(True story: Every time my build files and our Makefile tells me to try running make update-deps, I am actually confused for a few seconds.)

[paulproteus]

Sorry to hear about your make update-deps false alarm woes! +1 to checking with access().

[paulproteus]

I haven't done this yet, so I'll deassign. I hope to get to it, though, but if someone else beats me to it, go for it.