Non-existent variants do not cause a failure messge

[ralph@…]

I tried

sudo port install perl5 +perl5.18

not realising that variant +perl5.18 does not exist. Instead of giving an error message and asking to chose a valid variant, macports silently used the default variant:

--->  Computing dependencies for perl5
--->  Fetching archive for perl5
--->  Attempting to fetch perl5-5.12.4_0+perl5_12.darwin_13.noarch.tbz2 from http://mse.uk.packages.macports.org/sites/packages.macports.org/perl5
--->  Attempting to fetch perl5-5.12.4_0+perl5_12.darwin_13.noarch.tbz2.rmd160 from http://mse.uk.packages.macports.org/sites/packages.macports.org/perl5
--->  Installing perl5 @5.12.4_0+perl5_12
--->  Deactivating perl5 @5.12.4_0+perl5_16
--->  Cleaning perl5
--->  Activating perl5 @5.12.4_0+perl5_12
--->  Cleaning perl5

which in this case was certainly unhelpful to say the least, essentially downgrading my perl.

If a variant is missing, macports should stop and give an error.

[ryandesign (Ryan Carsten Schmidt)]

Duplicate of #14648.