Cleanup: Move package download/install code out of subproc.py

Instead do such things in bpkg, and only handle interfacing between
blender and bpkg in subproc.py

package_manager/bpkg/exceptions.py

@@ -7,5 +7,5 @@ class InstallException(BpkgException):
 class DownloadException(BpkgException):
     """Raised when there is an error downloading something"""
 
-class BadRepository(BpkgException):
-    """Raised when reading a repository results in an error"""
+class BadRepositoryException(BpkgException):
+    """Raised when there is an error while reading or manipulating a repository"""