option to make "error on other task" not fatal

[christophprokop]

sometimes you might want to keep building packages in a larger tree even though another part already failed an option for butido build like "--continue-on-error" could be helpful here when waiting time is not an issue

[ammernico]

We tested it and found that a package can be built and packed even if there are errors in other builds, as long as those errors do not impact the current package.

This behavior also occurs when building with a specified staging directory.

However, our test case was simple and had no dependencies. There might be something else wrong when building packages that depend on each other or share the same failing dependency.

[christophprokop]

additional note: this is mostly desired for METAPKG-$customer where several build trees do not depend on each other