Better error message when parsing data with BOM

If an input bytestring starts with a BOM, `aeson` will fail to parse it as JSON with the error message "Error in $: Failed reading: not a valid json value". Rejecting the input is correct per the spec, and as discussed in #389 it is better for the application to remove the BOM before parsing if needed, rather than having `aeson` do it automatically.

However, it would be nice if the error message was more explicit about the cause of the problem. At least that would have saved me a bit of confusion when I couldn't get `aeson` to parse a JSON file that looked perfectly fine in my editor. :-)