Auto merge of #396 - andrea-prearo:error-message-sub-expressions, r=B…

…urntSushi

Remove sub-expressions from parsing error

This PR skips printing sub-expressions in the error message.

This should avoid situations where the error message contains an AST with unreadable characters, as shown [here](BurntSushi/ripgrep#395).

<img width="1030" alt="unreadable-chars" src="https://user-images.githubusercontent.com/12599697/29924193-02147d0c-8e11-11e7-8b36-a97b63f802b4.png">

regex-syntax/src/lib.rs

@@ -1574,9 +1574,8 @@ impl fmt::Display for ErrorKind {
                 write!(f, "Invalid character class range '{}-{}'. \
                            Character class ranges must start with the smaller \
                            character, but {} > {}", start, end, start, end),
-            InvalidClassEscape(ref e) =>
-                write!(f, "Invalid escape sequence in character \
-                           class: '{}'.", e),
+            InvalidClassEscape(_) =>
+                write!(f, "Invalid escape sequence in character class."),
             InvalidRepeatRange { min, max } =>
                 write!(f, "Invalid counted repetition range: {{{}, {}}}. \
                            Counted repetition ranges must start with the \
@@ -1588,9 +1587,8 @@ impl fmt::Display for ErrorKind {
                 write!(f, "Missing maximum in counted repetition operator."),
             RepeaterExpectsExpr =>
                 write!(f, "Missing expression for repetition operator."),
-            RepeaterUnexpectedExpr(ref e) =>
-                write!(f, "Invalid application of repetition operator to: \
-                          '{}'.", e),
+            RepeaterUnexpectedExpr(_) =>
+                write!(f, "Invalid application of repetition operator."),
             UnclosedCaptureName(ref s) =>
                 write!(f, "Capture name group for '{}' is not closed. \
                            (Missing a '>'.)", s),