Unwrap exception and use sql/execute! instead of do-do-commands


1. Unwrap exception 

    Tries to replace the current exception with .getNextException in the SQL case.
    With the postgres adapater only getNextException will show something helpful. 

2. sql/execute!

    This enables to load sql-files like:

          (defn up [] [(slurp "migrations_sql/create-links.up.sql")])

    Imho sql-files are much more convenient for pure DDL statements.