Simply remving the escaping seems wrong as a ValueError might contain a
text which needs escaping, just imagine a detector contains the code
"x = int(foo)" and foo happens to contain "<br>":
ValueError: invalid literal for int() with base 10: '<br>'

I like the approach of providing a separate exception class, but the
name ValueErrorNoescape sounds wrong. Maybe something like
"RawErrorMessage"?

I don't know right now, but isn't the error message sent in the email
reply if one submits changes via the email interface? So different ways
of escaping (or not escaping) might be needed here. HTML links, links to
other issues etc. could be done in a similar way to
property.plain(hyperlink=1)
i.e. "issue123" gets converted to a link to the issue and
http://example.com/ will turn into a link to this URL.