Change Constant(undef) error to something meaningful

Change Constant(undef) error to something meaningful

Look at these errors:

$ perl5.16.0 -e 'use overload; BEGIN{overload::constant q => sub{}; } "aaa"'
Constant(q): Call to &{$^H{q}} did not return a defined value at -e line 1, near "} "aaa""
Execution of -e aborted due to compilation errors.

$ perl5.16.0 -e 'BEGIN{++$_ for @INC{<charnames.pm _charnames.pm>}} "\N{a}"'
Constant(\N{a}) unknown at -e line 1, within string
Execution of -e aborted due to compilation errors.

$ perl5.16.0 -e 'use overload; BEGIN{overload::constant q => sub{}; } tr"aaa""'
Constant(tr): Call to &{$^H{q}} did not return a defined value at -e line 1, within string
Execution of -e aborted due to compilation errors.

The (q) and (tr) might seem a bit odd, but are not completely meaning-
less.  They match the third argument passed to the overload handler.
Now look at this:

$ perl5.16.0 -e 'use overload; BEGIN{overload::constant integer => sub{}; } 123'
Constant(undef): Call to &{$^H{integer}} did not return a defined value at -e line 1, at end of line
Execution of -e aborted due to compilation errors.

$ perl5.16.0 -e 'use overload; BEGIN{overload::constant float => sub{}; } 1.23'
Constant(undef): Call to &{$^H{float}} did not return a defined value at -e line 1, at end of line
Execution of -e aborted due to compilation errors.

$ perl5.16.0 -e 'use overload; BEGIN{overload::constant binary => sub{}; } 0x123'
Constant(undef): Call to &{$^H{binary}} did not return a defined value at -e line 1, at end of line
Execution of -e aborted due to compilation errors.

That Constant(undef) is not helpful.  This commit changes it to show
the number itself, making these cases similar to \N{}.