configy: Catch and re-throw validation errors

[VPanteleev-S7]

The documentation states:

If validation of individual fields is not enough and the section as
a whole needs to be validated, one can implement a void validate()
const method which throws an exception in the even of a validation
failure. The library will rethrow this Exception with the
file/line information pointing to the section itself, and not any
individual field.

The emphasized section was not actually implemented - exceptions thrown from validate() functions bubbled up to the top level and were printed without any context.

Fix this by implementing the functionality.