Add error message for non-package main declaration

[jhavrane]

Fix #4140

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[jhavrane]

Btw format specifier %1$ on line 281 is most likely not correct.

...
    if (!main->is<IR::Declaration_Instance>()) {
        ::error(ErrorType::ERR_INVALID, "%1$: must be a package declaration", main->getNode());
...

I tried to print with this specifier and got the following exception thrown. Got boost version 1.71

terminate called after throwing an instance of 'boost::wrapexcept<boost::io::bad_format_string>'
  what():  boost::bad_format_string: format-string is ill-formed

[fruffy]

Btw format specifier %1$ on line 281 is most likely not correct.

...
    if (!main->is<IR::Declaration_Instance>()) {
        ::error(ErrorType::ERR_INVALID, "%1$: must be a package declaration", main->getNode());
...

I tried to print with this specifier and got the following exception thrown. Got boost version 1.71

terminate called after throwing an instance of 'boost::wrapexcept<boost::io::bad_format_string>'
  what():  boost::bad_format_string: format-string is ill-formed

This looks like a typo to me. Could you also fix it with this PR?

[jhavrane]

Fixed in #4145