Implement better error message for the v4

[peaBerberian]

Until now, the RxPlayer's errors set the Error's name property inside its message property.

When logged in the console, such errors are automatically formatted by the browser which generally choose to re-use the name to produce a meaningful error string.
This lead to a repeat like:
EncryptedMediaError: EncryptedMediaError (INCOMPATIBLE_KEYSYSTEMS) Some description message

This was unnecessary and kind of ugly but we were afraid some apps were for whatever reasons exploiting the message string and thus did not change it in the v3.

For the v4 we choose a more consize message: The code, followed by a colon, followed by a description of what happened. As such, the previous example would look like in a JS console when logged:
EncryptedMediaError: INCOMPATIBLE_KEYSYSTEMS: Some description message

If applications want to detect the error's name/type they can respectively rely on the more appropriate name or type property (basically the name is the PascalCased name here to have an idiomatic JS error, type is the RxPlayer's documented UPPER_SNAKE_CASE version of the name).

Note that the error's message is still not part of the API and applications should still expect that it could theoretically change its format at any time, even if we're generally (too?) careful with that sort of things.