SMM: Misleading exception message when required values not provided in connection string

Repro:

```
ShardMapManagerFactory.GetSqlShardMapManager("", ShardMapManagerLoadPolicy.Lazy)
```

Exception thrown:

```
The required property 'DataSource' must be set in the connection string.
Parameter name: connectionString"
```

This exception is a little misleading because the property in the connection string is "Data Source" (with a space).