improve pod panic msg

[tbrowder]

No description provided.

[AlexDaniel]

https://irclog.perlgeek.de/perl6/2018-02-10#i_15801644

[tbrowder]

I have the test file that shows the PR works, but at the moment it has to be executed manually by the generated nqp from this PR.

$ cat pod-test.nqp
=pod
text...
=cut

Execute the file:

$ ./nqp pod-test.nqp
Obsolete pod format (=pod/=cut), please use =begin/=end instead at line 5, near "\n"
   at gen/moar/stage2/NQPHLL.nqp:707  (NQPHLL.moarvm:panic)
 from gen/moar/stage2/NQP.nqp:876  (nqp.moarvm:)
...

Note the first line shows the desired improved panic msg.

See the gist https://gist.github.com/tbrowder/ec5ccce995e27b8f28fdf176ccb6e6a0 for the desired nqp wrapper test file and the resulting error.

[tbrowder]

Thanks!