The point of the "OrDefault" functions was originally to provide a
default value for if the configuration option was not present. The
original implementation returned the default if any errors occurred,
which could cause improper configurations to go unnoticed.

Now the default is only used if the interface is nil, otherwise the
error is returned.

Errors now contain the improper value, to give more context as to where
the error occurred.

Errors were originally based on constant strings so the error could be
checked against the string to see if that's the error it was. With calls
to fmt.Errorf, this no longer works, so there are new helper functions
to determine if an error is of a specfic type. These functions assume
only errors from interparser are given to them, so errors from elsewhere
may cause false positives.