Better highlight for repeat count error

Before:
````
test.rs:3:21: 3:30 error: expected constant integer for repeat count but found variable
test.rs:3             let a = ~[0, ..n]; //~ ERROR expected constant integer for repeat count but found variable
                              ^~~~~~~~~
````

After:
````
test.rs:3:27: 3:28 error: expected constant integer for repeat count but found variable
test.rs:3             let a = ~[0, ..n]; //~ ERROR expected constant integer for repeat count but found variable
                                     ^
````
auto merge of #5198 : youknowone/rust/repeat-count, r=brson

Before:
````
test.rs:3:21: 3:30 error: expected constant integer for repeat count but found variable
test.rs:3             let a = ~[0, ..n]; //~ ERROR expected constant integer for repeat count but found variable
                              ^~~~~~~~~
````

After:
````
test.rs:3:27: 3:28 error: expected constant integer for repeat count but found variable
test.rs:3             let a = ~[0, ..n]; //~ ERROR expected constant integer for repeat count but found variable
                                     ^
````