Auto merge of #3991 - alexcrichton:better-errors, r=matklad

Improve configuration load errors

Add some more context information on the backtrace

Author: bors

src/cargo/util/config.rs

@@ -394,7 +394,10 @@ impl Config {
 
         walk_tree(&self.cwd, |mut file, path| {
             let mut contents = String::new();
-            file.read_to_string(&mut contents)?;
+            file.read_to_string(&mut contents).chain_error(|| {
+                human(format!("failed to read configuration file `{}`",
+                              path.display()))
+            })?;
             let toml = cargo_toml::parse(&contents,
                                          &path,
                                          self).chain_error(|| {
@@ -405,7 +408,10 @@ impl Config {
                 human(format!("failed to load TOML configuration from `{}`",
                               path.display()))
             })?;
-            cfg.merge(value)?;
+            cfg.merge(value).chain_error(|| {
+                human(format!("failed to merge configuration at `{}`",
+                              path.display()))
+            })?;
             Ok(())
         }).chain_error(|| human("Couldn't load Cargo configuration"))?;
 

tests/bad-config.rs

@@ -120,6 +120,9 @@ fn bad5() {
 Caused by:
   Couldn't load Cargo configuration
 
+Caused by:
+  failed to merge configuration at `[..]`
+
 Caused by:
   failed to merge key `foo` between files:
   file 1: [..]foo[..]foo[..]config