[!24] Throw errors if unavailable versions are requested

Merge branch 'feature/errors-on-unavailable-versions' into 'master'

See merge request [spack/dune-spack!24]

  [spack/dune-spack!24]: Nonespack/dune-spack/merge_requests/24

packages/dune/package.py

@@ -242,6 +242,7 @@ class Dune(CMakePackage):
         branch='master',
         when='@master+gridglue',
     )
+    conflicts('dune@2.7', when='+gridglue')
 
     # The dune-python package migrated to dune-common after the 2.7 release
     resource(
@@ -258,6 +259,7 @@ class Dune(CMakePackage):
         branch='bugfix/library-build',
         when='@master+pdelab',
     )
+    conflicts('dune@2.7', when='+pdelab')
 
     # The dune-codegen package does not yet have a 2.7-compatible release
     resource(
@@ -267,6 +269,7 @@ class Dune(CMakePackage):
         when='@master+codegen',
         submodules=True,
     )
+    conflicts('dune@2.7', when='+codegen')
 
     # Make sure that Python components integrate well into Spack
     extends('python')