blob: include blob key in errors

[vangent]

I noticed this from an issue filed against Pulumi. It doesn't appear to be a Go CDK issue, but the error message wasn't very clear ("The specified blob does not exist") and the user noted that it "doesnt tell which blob it would expect and what content that should have."

Services appear to be inconsistent about whether they include the key name in "not found" errors (see below for details).

Since the portable type (gocloud.dev/blob) knows the key, we should just make sure it's included in the error message for all errors that apply to a key. We already include a prefix, so it's shouldn't be hard to add it there.

GCS:

blob (code=NotFound): storage: object doesn't exist

S3:

blob (code=NotFound): NoSuchKey: The specified key does not exist.
	status code: 404, request id: ...

Azure:

Read after failed write: blob (code=NotFound): -> github.com/Azure/azure-storage-blob-go/azblob.newStorageError, /usr/local/google/home/rvangent/go/pkg/mod/github.com/!azure/azure-storage-blob-go@v0.6.0/azblob/zc_storage_error.go:42
===== RESPONSE ERROR (ServiceCode=BlobNotFound) =====
Description=The specified blob does not exist.
RequestId:d68d0b9f-e01e-0107-36b4-092564000000
Time:2019-05-13T17:52:46.5463275Z, Details: 
   Code: BlobNotFound
   GET https://gocloudblobtests.blob.core.windows.net/go-cloud-bucket/blob-for-reading
   Authorization: REDACTED
   Content-Type: []
   User-Agent: [go-cloud/blob/0.1.0 Azure-Storage/0.6 (go1.12.5; linux)]
   X-Cloud-Trace-Context: [e2790d62e0779aaa44a51111597b5f38/16106888398616378301;o=0]
   X-Ms-Date: [Tue, 16 Jul 2019 15:41:08 GMT]
   X-Ms-Version: [2018-11-09]
   --------------------------------------------------------------------------------
   RESPONSE Status: 404 Not Found
   Content-Length: [215]
   Content-Type: [application/xml]
   Date: [Mon, 13 May 2019 17:52:45 GMT]
   Server: [Windows-Azure-Blob/1.0 Microsoft-HTTPAPI/2.0]
   X-Ms-Error-Code: [BlobNotFound]
   X-Ms-Request-Id: [d68d0b9f-e01e-0107-36b4-092564000000]
   X-Ms-Version: [2018-11-09]

[Other errors looked similar]

File:

blob (code=NotFound): stat /tmp/go-cloud-fileblob/blob-for-reading: no such file or directory