Unhelpful error message when there is a big list with duplicates

[Leo Arias]

When there are duplicates in a list in the yaml that must be unique, the error message is not helpful to find the culprits:

Issues while validating snapcraft.yaml: The 'build-packages' property does not match the required schema: ['git', 'build-essential', 'cmake', 'flex', 'bison', 'pyqt5-dev', 'qttools5-dev', 'qtpositioning5-dev', 'libqt5svg5-dev', 'libqt5webkit5-dev', 'libqt5gui5', 'libqt5scripttools5', 'qtscript5-dev', 'libqca-qt5-2-dev', 'grass-dev', 'libgeos-dev', 'libgdal-dev', 'libqt5xmlpatterns5-dev', 'libqt5scintilla2-dev', 'pyqt5.qsci-dev', 'python3-pyqt5.qsci', 'libgsl-dev', 'txt2tags', 'libproj-dev', 'libqwt-qt5-dev', 'libspatialindex-dev', 'pyqt5-dev-tools', 'qttools5-dev-tools', 'qt5-default', 'python3-future', 'python3-pyqt5.qtsql', 'python3-psycopg2', 'coreutils', 'python3-sip', 'libqt5sql5-sqlite', 'libqca-qt5-2-plugins', 'python3-gdal', 'qttools5-dev', 'python3-yaml'] has non-unique elements

Steps to reproduce:

$ snapcraft init
add to duplicate packages to build-packages.
$ snapcraft

The error message should mention the duplicate entries, so they can be easily removed.