add reason to bad connection error

[kuerbiskarton]

Instead of raising a plain driver.ErrBadConn error, raise an error that

• wraps driver.ErrBadConn.
• provides information why the connection is bad (by adding the text of the underlying error).

This is helpful in case of issues with server certificate validation. Instead of just

driver: bad connection

the error is then

driver: bad connection: x509: certificate signed by unknown authority
driver: bad connection: x509: certificate relies on legacy Common Name field, use SANs instead

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