Make InstallError and ExternalPackageError visiible in packages.

lib/spack/spack/__init__.py

@@ -189,5 +189,9 @@
 from spack.util.executable import *
 __all__ += spack.util.executable.__all__
 
-from spack.package import install_dependency_symlinks, flatten_dependencies, DependencyConflictError
-__all__ += ['install_dependency_symlinks', 'flatten_dependencies', 'DependencyConflictError']
+from spack.package import \
+    install_dependency_symlinks, flatten_dependencies, DependencyConflictError, \
+    InstallError, ExternalPackageError
+__all__ += [
+    'install_dependency_symlinks', 'flatten_dependencies', 'DependencyConflictError',
+    'InstallError', 'ExternalPackageError']

lib/spack/spack/package.py

@@ -1351,6 +1351,10 @@ def __init__(self, message, long_msg=None):
         super(InstallError, self).__init__(message, long_msg)
 
 
+class ExternalPackageError(InstallError):
+    """Raised by install() when a package is only for external use."""
+
+
 class PackageStillNeededError(InstallError):
     """Raised when package is still needed by another on uninstall."""
     def __init__(self, spec, dependents):