Update invalid sql_connection error message

Previously, the sql_connection validation error
message had two problems:

1. It did not print out the offending string
2. The error was a bit misleading, b/c the
validation check does more than just check the
db_type.

This commit addresses the above issues in order
to ensure that users can better understand this
error message if they run into it.

Change-Id: Id5944bf035a13ad353d1417b13307067243a1a3c