Make build errors more user-friendly

[viktorprogger]

Error messages are not verbose enough now. I.e. you can't examine why didn't this package be installed:

My proposal is to add some more information to the error text. In most cases current merging file path will be enough. It could also be great to show current merging position of the array if possible.

At least plugin should say that it was an error during a build, not during a package installation.

[viktorprogger]

E.g. something like that:

[ConfigBuildException]
An error occured during configuration build.
The file being processed: 'config/common.php'.
Error text:
    Undefined array key "yiisoft/db-mysql"
Please, take care of fixing errors in this file before you continue.

[samdark]

That would be awesome!