Throw new Errors rather than strings

Created by: davemevans

Whilst debugging some TTML streams I noticed that the parser logged any exceptions as undefined. Turns out this is because the parser throws strings and logging code was looking for e.message of an Error object which is obviously undefined if you throw a string. Fix was to change all string throws to new Error throws.

Throwing strings seems pretty bad form - Errors allow for stack tracing etc - so I changed all the other instances for good measure.