Improve error message when build.executable doesn't exist (#1303)

texlab outputs on standard error:

	ERROR - Failed to compile document "file:///path/to/test.tex":
	Unable to run compiler: No such file or directory (os error 2)

which is confusing because the actual issue is that "latexmk" was
not found.  Fix that by adding the program name.  This might not be
as appropriate for all possible io::Error kinds but it's probably
useful for a few of them.

crates/commands/src/build.rs

@@ -80,26 +80,27 @@ impl BuildCommand {
             self.working_dir.display()
         );
 
-        let mut process = self.spawn_internal()?;
+        let mut process = self.spawn_internal().map_err(|err| {
+            BuildError::from(std::io::Error::other(format!("{}: {}", self.program, err)))
+        })?;
         track_output(process.stderr.take().unwrap(), sender.clone());
         track_output(process.stdout.take().unwrap(), sender);
         Ok(process)
     }
 
     #[cfg(windows)]
-    fn spawn_internal(&self) -> Result<Child, BuildError> {
+    fn spawn_internal(&self) -> std::io::Result<Child> {
         std::process::Command::new(&self.program)
             .args(self.args.clone())
             .stdin(Stdio::null())
             .stdout(Stdio::piped())
             .stderr(Stdio::piped())
             .current_dir(&self.working_dir)
             .spawn()
-            .map_err(Into::into)
     }
 
     #[cfg(unix)]
-    fn spawn_internal(&self) -> Result<Child, BuildError> {
+    fn spawn_internal(&self) -> std::io::Result<Child> {
         use std::os::unix::process::CommandExt;
         std::process::Command::new(&self.program)
             .args(self.args.clone())
@@ -109,7 +110,6 @@ impl BuildCommand {
             .current_dir(&self.working_dir)
             .process_group(0)
             .spawn()
-            .map_err(Into::into)
     }
 
     #[cfg(windows)]