fix malformed warning to print the way it was meant to print
fix malformed warning to print the way it was meant to print

Given a meson.build with the contents:

```
t = '
'
```

We want to warn that this is bad. So we emitted this warning:
```
WARNING: Newline character in a string detected, use ''' (three single quotes) for multiline strings instead.
This will become a hard error in a future Meson release.                                 t = ' 4 4
```

The line contents and the offset are printed as gibberish after a big
whitespace run. These are elsewhere often passed to ParseException,
which pretty-prints this, but newlines aren't an exception, merely a
warning, and mlog.warning doesn't accept numeric strings as anything
more meaningful than something to print as text.

Fix this (by wrapping it in a ParseException) to properly print:
```
meson.build:4: WARNING: Newline character in a string detected, use ''' (three single quotes) for multiline strings instead.
This will become a hard error in a future Meson release.
t = '
    ^
```

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