BUG: jplspec: max_lines warning is stored in metadata and therefore hidden from users

For queries that has more lines than the max_lines, the logic is broken. The warning about hitting the limit is parsed out from the response table, and is stored in the metadata of the result table, and is not hit currently, which is not surprising as the server returns only maxlines number of lines, plus the warning that we parse out.

```
if len(result) > self.maxlines:
warnings.warn("This form is currently limited to {0} lines."
"Please limit your search.".format(self.maxlines))
```

Also, while I see that the we just try to reraise the warning coming from the service, I find the actual warning message confusing. What does `Please limit your search` try to actually mean?

```
>>> from astroquery.jplspec import JPLSpec
>>> import astropy.units as u
>>> result = JPLSpec.query_lines(min_frequency=100 * u.GHz,
... max_frequency=1000 * u.GHz,
... min_strength=-500,
... molecule="H2O",
... max_lines=10,
... parse_name_locally=True)

>>> result.meta
OrderedDict([('comments',
['',
'',
'',
'form is currently limilted to 10 lines. Please limit your search.'])])
```

Start at astroquery.jplspec.JPLSpec.query_lines and trace how the max_lines response warning is parsed and placed in result.meta. Reproduce the query shown with max_lines=10, then inspect the related JPLSpec tests if present. Done means the limit warning is surfaced to users clearly rather than being hidden in metadata, with wording that explains the required action.

Written by the indexing model from the issue text.