Fail build when there are errors

[byrond]

Currently, when the artifact build or style guide install fails, they do not generate an error that fails the build. This makes it look like all steps completed successfully on CI when there were actually errors.

• Sets haltonfailure to true for the call to artifact-main to make sure errors will stop the build.

• TODO: Verify style guide build errors halt the build

Testing

• composer require palantirnet/the-build:dev-fail-on-build-errors
• Update the following line in your project's build.xml file:
  <phing phingfile="build.xml" target="artifact-main" inheritAll="false">
  to read:
  <phing phingfile="build.xml" target="artifact-main" inheritAll="false" haltonfailure="true">
• Induce a styleguide build error.
• phing artifact
• Verify the artifact build fails.
• Fix the styleguide build error, and induce some other build error.
• phing artifact
• Verify the artifact build fails.