If reading secrets from BlobStorage fails, log the reason.

Currently, if we failed to read secrets from blog storage, we do not log reason. Fixing this will help better diagnose future failures.

https://github.com/Azure/azure-functions-host/blob/dev/src/WebJobs.Script.WebHost/Security/KeyManagement/BlobStorageSecretsRepository.cs#L93

```
public override async Task ReadAsync(ScriptSecretsType type, string functionName)
{
string secretsContent = null;
string blobPath = GetSecretsBlobPath(type, functionName);
try
{
CloudBlockBlob secretBlob = Container.GetBlockBlobReference(blobPath);
if (await secretBlob.ExistsAsync())
{
secretsContent = await secretBlob.DownloadTextAsync();
}
}
catch (Exception e)
{
LogErrorMessage("read");
throw e;
}

return string.IsNullOrEmpty(secretsContent) ? null : ScriptSecretSerializer.DeserializeSecrets(type, secretsContent);
}
````