fix(error): Report context for spawmn failures

Author: epage

src/assert.rs

@@ -1,4 +1,5 @@
 use environment::Environment;
+use error_chain::ChainedError;
 use errors::*;
 use output::{OutputAssertion, OutputKind};
 use std::default;
@@ -308,7 +309,9 @@ impl Assert {
             None => command,
         };
 
-        let mut spawned = command.spawn()?;
+        let mut spawned = command
+            .spawn()
+            .chain_err(|| ErrorKind::SpawnFailed(self.cmd.clone()))?;
 
         if let Some(ref contents) = self.stdin_contents {
             spawned
@@ -365,7 +368,7 @@ impl Assert {
     /// ```
     pub fn unwrap(self) {
         if let Err(err) = self.execute() {
-            panic!("{}", err);
+            panic!("{}", err.display_chain());
         }
     }
 }

src/errors.rs

@@ -6,6 +6,14 @@ error_chain! {
         Fmt(::std::fmt::Error);
     }
     errors {
+        SpawnFailed(cmd: Vec<String>) {
+            description("Spawn failed")
+            display(
+                "{}: (command `{}` failed to run)",
+                ERROR_PREFIX,
+                cmd.join(" "),
+            )
+        }
         StatusMismatch(cmd: Vec<String>, expected: bool, out: String, err: String) {
             description("Wrong status")
             display(