Parsing error messages not always helpful

[SimonSapin]

Reduced test case:

syn::parse_expr("match foo { Some(a) => a, , None => 0 }").unwrap()

Output:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: "failed to parse tokens after expression: \" foo { Some(a) => a, , None => 0 }\""', ../src/libcore/result.rs:799

The error message says that the part of the input that failed to parse is: all of it, except for the initial match keyword. The message is similar when parsing a much larger match expression, with the syntax error somewhere in the middle of it.

[dtolnay]

Triaging as out of scope for 0.12 but a major focus for 0.13.

[dtolnay]

Let's continue to track this as part of #47.