Fix panics caused by unregistered algorithm

[Wwwsylvia]

We need to scan the oras-go code base to identify and fix any instances where an unregistered algorithm could cause a panic.

I just found an instance:

oras-go/content/reader.go

Lines 100 to 111 in d92df9d

	// NewVerifyReader wraps r for reading content with verification against desc.
	func NewVerifyReader(r io.Reader, desc ocispec.Descriptor) *VerifyReader {
	verifier := desc.Digest.Verifier()
	lr := &io.LimitedReader{
	R: io.TeeReader(r, verifier),
	N: desc.Size,
	}
	return &VerifyReader{
	base: lr,
	verifier: verifier,
	}
	}

In this function, if the algorithm used in the digest is unregistered, line 102 will panic.

I also tested OCIStore.Fetch(), Repository.Fetch(), and Repository.FetchReference(). I observed that if there is an unsupported digest type is returned in the "Docker-Content-Digest" header, oras-go will return an error instead of panicking.