In the grammar scanner, STRING_FINISH unclosed constructs and return

In the grammar scanner, STRING_FINISH unclosed constructs and return
them to the parser in order to improve error messages.
* src/scan-gram.l (SC_ESCAPED_STRING, SC_ESCAPED_CHARACTER,
SC_BRACED_CODE, SC_PROLOGUE): Implement.
* tests/input.at (Unclosed constructs): New test case.
* tests/regression.at (Invalid inputs): Update now that unclosed %{ is
seen.

* src/scan-gram.h, src/scan-gram.l (gram_last_braced_code_loc): Remove
unused global.