Improve error messages from editconf and trjconv

Fail properly if a tpr file is not supplied when using the -conect option.

Also avoid a "File input/output error:" when no tpr file is supplied when it is required for -pbc options.

The editconf changes were largely introduced in 102447a5 but overwritten shortly after in a branch merge dac65bb1.

Fixes: #5032 (closed)