Improve mypy --pretty output for (multiline) f-strings

Error messages merely say the line number, which is not always accurate - e.g. for multiline strings it's merely the first or last line of the overall multiline string (depending on Python version). If it's a format string and there's dozens of {} elements embedded in it, maybe across hundreds of lines, it can be practically impossible to tell what mypy is referring to. e.g. here's the actual output from a current problem:

     src.py:136: error: "str" has no attribute "expression"  [attr-defined]
     src.py:136: error: Item "str" of "Optional[str]" has no attribute "expression"  [union-attr]
     src.py:136: error: Item "None" of "Optional[str]" has no attribute "expression"  [union-attr]
     src.py:136: error: Item "float" of "Optional[float]" has no attribute "expression"  [union-attr]
     src.py:136: error: Item "None" of "Optional[float]" has no attribute "expression"  [union-attr]
     src.py:136: error: "float" has no attribute "expression"  [attr-defined]
     src.py:136: error: "bool" has no attribute "expression"  [attr-defined]
     src.py:136: error: "int" has no attribute "expression"  [attr-defined]

The multiline string these are embedded in is nearly a hundred lines long and has dozens of embedded elements, almost all of which reference an espression attribute. I genuinely have no idea which ones these messages are referring to. If it at least told me the name of the variable (or whatever), I'd be able to narrow it down.

Start by tracing mypy's --pretty diagnostic rendering for errors arising in multiline f-strings; compare the reported location with the embedded expression that triggered it. Done means pretty output identifies the relevant f-string expression or variable rather than only the overall string's line.