Improve the error management when /proc is not mounted

[sylvestre]

This PR does two things:

• Triggers an error on GNU/Linux & Android when /proc/self/exe doesn't exist
• Handle the error properly

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @arielb1 (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[aidanhs]

Thanks for the PR @sylvestre, we'll make sure it gets reviewed by @arielb1 or another reviewer soon!

[arielb1]

r? @alexcrichton

[alexcrichton]

@bors: r+

Thanks @sylvestre!

[bors]

📌 Commit 1b6a182 has been approved by alexcrichton

[bors]

⌛ Testing commit 1b6a182 with merge 561a121...

[bors]

💔 Test failed - status-appveyor

[sylvestre]

Probably not my fault:

Downloading rustc-serialize v0.3.24
error: unable to get packages from source
To learn more, run the command again with --verbose.
Build completed unsuccessfully in 0:02:28

[alexcrichton]

@bors: retry

• appveyor network issues Tracking issue for spurious network failures on bots #40474