Produce helpful error message when requesting an unsupported or unavailable `geo_type` or `time_type` in `Epidata$covidcast`

[brookslogan]

Epidata$covidcast("nchs-mortality","deaths_allcause_incidence_num","epiweek","state",202101,"ca")

and

Epidata$covidcast("jhu-csse","confirmed_incidence_num","week","state",202101,"ca")

and

Epidata$covidcast("jhu-csse","confirmed_incidence_num","day","sstate",20210101,"ca")

all produce

$result
[1] -2

$message
[1] "no results"

The first query requested "epiweek" data rather than "week" data, the second requested "week" data when the source&signal does not have any available, and the third contained a typo in the geo_type.

It would be nice to have messages along the lines of:

• "invalid time_type"
• "requested signal does not support the requested time_type and geo_type"
• "invalid geo_type"

[melange396]

If we want to handle this client-side, available time_types and geo_types for each signal can be found via the "metadata" endpoint(s). They are enumerated as part of the metadata generation, and this information is recomputed regularly.