Return informative get_eurostat_json error messages to the user

[pitkant]

Eurostat website lists different types of HTTP status errors that Statistics API can return. As mentioned in #260 these error messages are not returned to the end user in R but they can be viewed e.g. in the web browser. It would be good if these ids (fault codes) and labels (descriptions) would be returned to the user in R as well.

Fault code	HTTP status	Description
Client Errors
100 No results found	400 Bad request	The result from the query is empty.
100 No results found	404 Not Found	The requested resource is not available.
140 Syntax error	400 Bad request	The query is invalid.
150 Semantic error	400 Bad request	The request is syntactically correct but fails a semantic validation, business rules.
Server errors
500 Internal Server error	500 Internal server error	The webservice should return this error code when none of the other error codes better describes the reason for the failure of the service to provide a meaningful response.Also used in case of request not respecting the xsd definition.

[pitkant]

Solved in #262 and d74bcb7