Report the actual error when failing to get the rustc version

This should help with pinpointing the issue in rust-lang/crater#663.

build.rs

@@ -6,7 +6,7 @@ fn main() {
     // Avoid unnecessary re-building.
     println!("cargo:rerun-if-changed=build.rs");
 
-    let (rustc_minor_ver, is_nightly) = rustc_minor_nightly().expect("Failed to get rustc version");
+    let (rustc_minor_ver, is_nightly) = rustc_minor_nightly();
     let rustc_dep_of_std = env::var("CARGO_FEATURE_RUSTC_DEP_OF_STD").is_ok();
     let align_cargo_feature = env::var("CARGO_FEATURE_ALIGN").is_ok();
     let const_extern_fn_cargo_feature = env::var("CARGO_FEATURE_CONST_EXTERN_FN").is_ok();
@@ -112,23 +112,27 @@ fn main() {
     }
 }
 
-fn rustc_minor_nightly() -> Option<(u32, bool)> {
+fn rustc_minor_nightly() -> (u32, bool) {
     macro_rules! otry {
         ($e:expr) => {
             match $e {
                 Some(e) => e,
-                None => return None,
+                None => panic!("Failed to get rustc version"),
             }
         };
     }
 
     let rustc = otry!(env::var_os("RUSTC"));
-    let output = otry!(Command::new(rustc).arg("--version").output().ok());
+    let output = Command::new(rustc)
+        .arg("--version")
+        .output()
+        .ok()
+        .expect("Failed to get rustc version");
     let version = otry!(str::from_utf8(&output.stdout).ok());
     let mut pieces = version.split('.');
 
     if pieces.next() != Some("rustc 1") {
-        return None;
+        panic!("Failed to get rustc version");
     }
 
     let minor = pieces.next();
@@ -144,7 +148,7 @@ fn rustc_minor_nightly() -> Option<(u32, bool)> {
         .unwrap_or(false);
     let minor = otry!(otry!(minor).parse().ok());
 
-    Some((minor, nightly))
+    (minor, nightly)
 }
 
 fn which_freebsd() -> Option<i32> {