Slightly improve an error message

This reminds me to review the error reporting and make it more
consistent. The InnerError should ideally not carry data at all but be
concrete error cases we want to set as contexts.

Author: killercup

src/error.rs

@@ -29,6 +29,9 @@ impl Display for Error {
 
 #[derive(Fail, Debug)]
 enum InnerError {
+    #[fail(display = "{}", _0)]
+    Custom(Context<String>),
+
     #[fail(display = "IO error: {}", _0)]
     Io(io::Error),
 
@@ -66,6 +69,22 @@ impl Error {
     }
 }
 
+impl From<InnerError> for Error {
+    fn from(kind: InnerError) -> Error {
+        Error {
+            inner: Context::new(kind),
+        }
+    }
+}
+
+impl From<Context<String>> for Error {
+    fn from(inner: Context<String>) -> Error {
+        Error {
+            inner: Context::new(InnerError::Custom(inner)),
+        }
+    }
+}
+
 impl From<io::Error> for Error {
     fn from(x: io::Error) -> Self {
         Error {

src/json.rs

@@ -1,6 +1,7 @@
 //! JSON output
 
 use crate::{Error, Target};
+use failure::ResultExt;
 use serde::Serialize;
 use serde_json::to_vec as write_json;
 use std::io::Write;
@@ -23,7 +24,11 @@ pub fn file<T: AsRef<Path>>(name: T) -> Result<Target, Error> {
         use std::io::BufWriter;
 
         let target = if path.exists() {
-            let mut f = OpenOptions::new().write(true).append(true).open(&path)?;
+            let mut f = OpenOptions::new()
+                .write(true)
+                .append(true)
+                .open(&path)
+                .with_context(|_| format!("Can't open file `{}` as JSON target", path.display()))?;
             f.write_all(b"\n")?;
 
             f