Skip invalid template files when loading workspace

[jrray]

For the use case of spk info --variants <valid spec file>, this command would fail with an error about a different file if another file exists with a name matching *.spk.yaml but loading this other file fails for any reason.

Like #1190 and #1191, this command is expected to succeed and is not intended to interact with any other files other than the one named on the command line.

[jrray]

This represents another failure mode we encountered at SPI. Someone was using a file with the .spk.yaml extension for a spawn target spec and now it was impossible to use spk info --variants <filename> for any valid spk recipe file in that directory.

Skipping invalid files seems like a good course of action, however if there is a use case for making this kind of thing a hard error then it would be best to add a test that demonstrates that use case. We can make warning vs erroring a runtime decision based on context.

An unintended consequence of this change is that now if when doing spk info --variants demo.spk.yaml and demo.spk.yaml has some problem that makes it fail to load, we'll first say we didn't find that recipe template:

This is a little better than literally saying "file not found" but perhaps it could say something like "did not find a valid recipe template" since the rest of the error goes on to say more specific things about a file.