Add better error messages for invalid globs.

This threads the original glob given by end users through all of the
glob parsing errors. This was slightly trickier than it might appear
because the gitignore implementation actually modifies the glob before
compiling it. So in order to get better glob error messages everywhere,
we need to track the original glob both in the glob parser and in the
higher-level abstractions in the `ignore` crate.

Fixes #444

ignore/src/lib.rs

@@ -112,7 +112,17 @@ pub enum Error {
     /// An error that occurs when doing I/O, such as reading an ignore file.
     Io(io::Error),
     /// An error that occurs when trying to parse a glob.
-    Glob(String),
+    Glob {
+        /// The original glob that caused this error. This glob, when
+        /// available, always corresponds to the glob provided by an end user.
+        /// e.g., It is the glob as writtein in a `.gitignore` file.
+        ///
+        /// (This glob may be distinct from the glob that is actually
+        /// compiled, after accounting for `gitignore` semantics.)
+        glob: Option<String>,
+        /// The underlying glob error as a string.
+        err: String,
+    },
     /// A type selection for a file type that is not defined.
     UnrecognizedFileType(String),
     /// A user specified file type definition could not be parsed.
@@ -144,7 +154,7 @@ impl Error {
             Error::WithDepth { ref err, .. } => err.is_io(),
             Error::Loop { .. } => false,
             Error::Io(_) => true,
-            Error::Glob(_) => false,
+            Error::Glob { .. } => false,
             Error::UnrecognizedFileType(_) => false,
             Error::InvalidDefinition => false,
         }
@@ -199,7 +209,7 @@ impl error::Error for Error {
             Error::WithDepth { ref err, .. } => err.description(),
             Error::Loop { .. } => "file system loop found",
             Error::Io(ref err) => err.description(),
-            Error::Glob(ref msg) => msg,
+            Error::Glob { ref err, .. } => err,
             Error::UnrecognizedFileType(_) => "unrecognized file type",
             Error::InvalidDefinition => "invalid definition",
         }
@@ -227,7 +237,10 @@ impl fmt::Display for Error {
                           child.display(), ancestor.display())
             }
             Error::Io(ref err) => err.fmt(f),
-            Error::Glob(ref msg) => write!(f, "{}", msg),
+            Error::Glob { glob: None, ref err } => write!(f, "{}", err),
+            Error::Glob { glob: Some(ref glob), ref err } => {
+                write!(f, "error parsing glob '{}': {}", glob, err)
+            }
             Error::UnrecognizedFileType(ref ty) => {
                 write!(f, "unrecognized file type: {}", ty)
             }