Better error message when validate() requires an unspecified option

Hello again,

I changed an option name in my @spec and forgot to change it in validate(requires => [...]).
Before this commit, that produced the following output:
```
Use of uninitialized value in concatenation (.) or string at ../getopt-lucid/Getopt-Lucid-1.06/lib/Getopt/Lucid.pm line 239.
Required option '' not found
```
With this commit:
```
Requiring an unspecified option ('foo') in validate()
```
Besides, this is not a `G::Lucid::Exception::ARGV` anymore (was catchable as
a mere user command-line mistake) but a `G::Lucid::Exception::Spec`. (I was not sure what was best between that and `G::Lucid::Exception::Usage`, but after all that's more about invalid specification than invalid method call.)

Cheers,