Replaced assertions with exceptions in database code, and changed

exception handling so that database connections aren't closed with
transactions in progress - this should make it possible to close the
database cleanly if an exception occurs.

components/net/sf/briar/db/DbStateException.java

@@ -0,0 +1,9 @@
+package net.sf.briar.db;
+
+import java.sql.SQLException;
+
+/** Thrown when the database is in an illegal state. */
+class DbStateException extends SQLException {
+
+	private static final long serialVersionUID = 10793396057218891L;
+}