Improve the error management when /proc is not mounted

This PR does two things:
* Triggers an error on GNU/Linux & Android when /proc/self/exe doesn't exist
* Handle the error properly

src/librustc/session/filesearch.rs

@@ -159,11 +159,16 @@ pub fn get_or_default_sysroot() -> PathBuf {
         })
     }
 
-    match canonicalize(env::current_exe().ok()) {
-        Some(mut p) => { p.pop(); p.pop(); p }
+    match env::current_exe() {
+        Ok(exe) => {
+            match canonicalize(Some(exe)) {
+                Some(mut p) => { p.pop(); p.pop(); return p; },
                 None => bug!("can't determine value for sysroot")
             }
         }
+        Err(ref e) => panic!(format!("failed to get current_exe: {}", e))
+    }
+}
 
 // The name of the directory rustc expects libraries to be located.
 fn find_libdir(sysroot: &Path) -> Cow<'static, str> {

src/libstd/sys/unix/os.rs

@@ -253,7 +253,12 @@ pub fn current_exe() -> io::Result<PathBuf> {
 
 #[cfg(any(target_os = "linux", target_os = "android", target_os = "emscripten"))]
 pub fn current_exe() -> io::Result<PathBuf> {
-    ::fs::read_link("/proc/self/exe")
+    let selfexe = PathBuf::from("/proc/self/exe");
+    if selfexe.exists() {
+        ::fs::read_link(selfexe)
+    } else {
+        Err(io::Error::new(io::ErrorKind::Other, "no /proc/self/exe available. Is /proc mounted?"))
+    }
 }
 
 #[cfg(any(target_os = "macos", target_os = "ios"))]