Improve error message for bogus -MO=… arguments

Improve error message for bogus -MO=… arguments

Commit 7a9b44b9 expanded the scope of the string eval that loads the
B::* backend module, but didn't move the $@ check and croak to outside
it.  Restore it and further improve the error message.

Before:

    $ perl -MO=Concise=-debug -e1
    syntax error at (eval 2) line 18, near "="
    BEGIN failed--compilation aborted.

After:

    $ ./perl -Ilib -MO=Concise=-debug -e1
    Loading compiler backend 'B::Concise=-debug' failed: syntax error at (eval 2) line 18, near "="
     at -e line 0.
    BEGIN failed--compilation aborted.