docs: tidy up the manage resources guide

[tonyandrewmeyer]

A quality pass over the "How to manage resources" guide, which had collected a few things: the example handler was a module-level function that nonetheless took self, the status message had a misplaced quote (resource 'my-resource; run ... for more info'), there was a comment musing about whether to reraise, logger.error(e) where the rest of the guides use logger.exception, and open(resource_path, 'r') on something fetch() returns as a Path.

I've also swapped the two except blocks so NameError comes first. Resources.fetch checks the name before it calls resource_get, and its docstring lists NameError first, so the guide was reading in the opposite order to the implementation. The sentence after the block now says what each exception actually means, rather than the previous single "does not exist" clause that only covered one of them.

The unit-test example used a foo OCI-image resource while the charmcraft.yaml above it declares a my-resource file resource, so I've made those match. One leftover > See first: blockquote in the integration-tests section is converted too - #2666 swept the file, but #2662 added that section afterwards.

Preview.

Fixes #2490

[tonyandrewmeyer]

@dwilding I've fixed the 'bugs' on the page, but I'm not sure this covers the 'quality' that the issue is asking for. Did you have specific things in mind? Should we be talking more about what you use resources for, or would you know that by the time you get to the how-to guide? I think there's a distinct difference between K8s and machine, should this take that into account more strongly? Might be a good one to talk over in our 1-1?