Refactor and some additions

[emphasize]

fixes:

• accidentally translated mustached variables in dialogs
• message type when fetching the system volume volume.get -> mycroft.volume.get

refactor:

• dialog naming
• adapt intents

additions:

• error dialog (if system volume couldn't be fetched)
• possibility to incrementally increase/decrease volume by 10% when asking for a relative change (omitting "by x percent", i.e. "increase volume" or simply "louder")
• get_response when asking for a total change and not mentioning the value (with validator)

tested (german)

[emphasize]

Will add additional translations (to the existing vocs) in a while --done

[emphasize]

Just realized nl-nl is missing --done