changed all 'already exists' errors to warnings

[speleophysics]

I was running into problems with using the NASA wmts service and came across these errors. Similar errors were pointed out in #619, which was fixed by pull request #656. However, I find that all of the "already exists" error messages in wmts.py were potentially problematic for the new configuration of the NASA server. Here I have converted all "already exists" errors into warnings, analogous to the change made in #656 for one of these error messages.

[greglucas]

Looks like this would close #768.

[erin-raif]

I'm also having the same issue and this would fix it.

[lgolston]

Yes, I'd like to echo this issue. Simply running:

from owslib.wmts import WebMapTileService
wmts = WebMapTileService("http://map1c.vis.earthdata.nasa.gov/wmts-geo/wmts.cgi")

Gives
KeyError: 'Content metadata for layer "MODIS_Combined_MAIAC_L2G_AerosolOpticalDepth" already exists'

We are seeing this over at cartopy, where the WMTS interface is currently broken as a result.

[greglucas]

This would be nice to get in to avoid these recurring errors that happen when NASA updates its server content. This does seem like an issue with the NASA servers and not here, but this would at least allow programs to keep running and just warn about bad data rather than erroring on bad data.

I sent an e-mail to NASA support about this issue as well to try and make them aware of it on their end.

[greglucas]

Ping again to try and get this looked at.

[github-actions]

This Pull Request has been inactive for 90 days. In order to manage maintenance burden, it will be automatically closed in 7 days.

[rcomer]

It would be great if this PR could get a maintainer decision, rather than being closed by the bot.

[geographika]

This looks fine to me to merge. Several other modules use warnings rather than errors.
Unless any objections @tomkralidis I can merge this in the next few days.

[geographika]

Thanks @speleophysics - merging.