Replaced InvalidCollationException (runtime) with proper UnssupportedCollationException
Updated UnsupportedCharsetException, so it returns an informative message, also in case an unsupported charset is used
merge SytzeH: Throw consistent exceptions for unsupported collations

Merge pull request #1259 from SytzeH/sql-layer/collation
https://github.com/FoundationDB/sql-layer/pull/1259