Subpar span on multiline format string with unclosed brace

[estebank]

Given a format string with an unclosed opening brace, the span points to the next element where a closing brace is expected. If this happens in a multiline format string, the span might be pointing to the line after the problem, with no secondary span referencing the opening brace. Either modify the error to be about "unclosed opening brace" pointing at the opening brace or add a secondary span pointing at it.

Introduced in #52649. CC #53836.