Fix README.md instructions and raise errors when LMP market not supported

[gsheni]

• Noticed some errors in the README (while copying and pasting codeblocks)
• Raise errors when LMP market not supported

[gsheni]

• One weird error I get locally (Mac) but not on the GitHub CI (Ubuntu) is the following:

        date_str = "20220722"
        for m in markets:
            print(iso.iso_id, m)
            latest = iso.get_latest_lmp(m, nodes=nodes)
>           assert isinstance(latest, pd.DataFrame)

isodata/tests/test_isos.py:251:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
isodata/miso.py:102: in get_latest_lmp
    time = pd.to_datetime(time_str).tz_localize(self.default_timezone)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

>   ???
E   TypeError: Cannot localize tz-aware Timestamp, use tz_convert for conversions

pandas/_libs/tslibs/timestamps.pyx:1849: TypeError

• Not sure if others are also running into this error. My guess is that GitHub machine is in a different timezone than my local computer

[kmax12]

the timezone error is interesting. filled a separate issue to take a look, but we can merge this is since it's unrelated