Dump current parse position when an error occurs.

[rjpower]

Hey Yang,

This just changes the parser to dump the current position of the parse when it encounters an error, rather then the Python stack:

../external/simple-rpc/rpc/rpcgen.py ../sparrow/sparrow_service.rpc
<f.0>:40:7: please use i32 or i64 for any integer types
>    bool done;
>        ^
while parsing type():
>    bool done;
>        ^
while parsing struct_field():
>    bool done;
>        ^
while parsing struct_fields():
>  struct IteratorResp {
>                       ^
while parsing struct_decl():
>  struct IteratorResp {
>        ^
while parsing structs_and_services():

[santazhang]

Thanks Russell! :D