radicle/cob: Include command name on spawn fail

To improve the ability to work out what went wrong, should an external
cob helper not be found, report the command name if the spawn fails.

Signed-off-by: Daniel Silverstone <dsilvers@digital-scurf.org>
Co-authored-by: Lorenz Leutgeb <lorenz.leutgeb@radicle.dev>

crates/radicle/src/cob/external.rs

@@ -114,6 +114,8 @@ pub enum Error {
     Op(#[from] OpEncodingError),
     #[error("serde_json: {0}")]
     Serde(#[from] JsonError),
+    #[error("failed to spawn program '{program}': {source}")]
+    Spawn { program: String, source: IoError },
     #[error("io: {0}")]
     Io(#[from] IoError),
 }
@@ -158,10 +160,14 @@ impl External {
             prefix + suffix
         };
 
-        let mut child = Command::new(command_name)
+        let mut child = Command::new(&command_name)
             .stdin(Stdio::piped())
             .stdout(Stdio::piped())
-            .spawn()?;
+            .spawn()
+            .map_err(|source| Error::Spawn {
+                program: command_name,
+                source,
+            })?;
 
         let stdin = child.stdin.take().expect("handle preset");