Improve error message when failing to run graphviz stuff

Fixes #682

cli/src/util.rs

@@ -1,3 +1,4 @@
+use super::error::{Error, Result};
 use std::io;
 use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::Arc;
@@ -31,12 +32,12 @@ pub struct LogSession();
 pub struct LogSession(PathBuf, Option<Child>, Option<ChildStdin>);
 
 #[cfg(windows)]
-pub fn log_graphs(_parser: &mut Parser, _path: &str) -> std::io::Result<LogSession> {
+pub fn log_graphs(_parser: &mut Parser, _path: &str) -> Result<LogSession> {
     Ok(LogSession())
 }
 
 #[cfg(unix)]
-pub fn log_graphs(parser: &mut Parser, path: &str) -> std::io::Result<LogSession> {
+pub fn log_graphs(parser: &mut Parser, path: &str) -> Result<LogSession> {
     use std::io::Write;
 
     let mut dot_file = std::fs::File::create(path)?;
@@ -46,11 +47,13 @@ pub fn log_graphs(parser: &mut Parser, path: &str) -> std::io::Result<LogSession
         .stdin(Stdio::piped())
         .stdout(dot_file)
         .spawn()
-        .expect("Failed to run Dot");
+        .map_err(Error::wrap(|| {
+            "Failed to run the `dot` command. Check that graphviz is installed."
+        }))?;
     let dot_stdin = dot_process
         .stdin
         .take()
-        .expect("Failed to open stdin for Dot");
+        .ok_or_else(|| Error::new("Failed to open stdin for `dot` process.".to_string()))?;
     parser.print_dot_graphs(&dot_stdin);
     Ok(LogSession(
         PathBuf::from(path),