Bugs/inconsistent validation behavior 179304918

[rosalinekarr]

Problem

Previously, announcements.isTombstoneAnnouncement() threw errors like the activity content validations which is inconsistent with the behavior of the other announcement validations. #179304918

Solution

Updated announcements.isTombstoneAnnouncement() to return false and updated content porcelain accordingly so it could still throw specific errors regarding faulty user arguments.

Change summary:

• Changed content.react() to throw InvalidEmojiStringError when passed an invalid emoji string
• Changed content.tombstone() to throw InvalidTombstoneAnnouncementTypeError or InvalidTombstoneAnnouncementSignatureError depending on which aspect of the provided target is invalid instead of simply throwing InvalidTombstoneAnnouncementTypeError for both
• Exported announcement.isValidEmoji() for testing emoji strings
• Exported announcement.isValidSignature() for testing signature strings
• Added announcement.isTombstoneableType() for testing if a given announcement type can be tombstoned
• Updated content.react() to throw InvalidAnnouncementUriError as specified in its documentation
• Added appropriate tests and documentation for above