fix(assert): Be explicit about spawn failure

Users had to infer the reason for the panic based on the location.
Instead, we should tell them what failed so they can more quickly get up
and going on fixing it.

Fixes assert-rs#109

Author: Ed Page

src/assert.rs

@@ -57,7 +57,12 @@ impl OutputAssertExt for process::Output {
 
 impl<'c> OutputAssertExt for &'c mut process::Command {
     fn assert(self) -> Assert {
-        let output = self.output().unwrap();
+        let output = match self.output() {
+            Ok(output) => output,
+            Err(err) => {
+                panic!("Failed to spawn {:?}: {}", self, err);
+            }
+        };
         Assert::new(output).append_context("command", format!("{:?}", self))
     }
 }