pmb.helpers.exceptions: Introduce (MR 2279)

Initially I thought of putting this in pmb/helpers/other.py, but this
resulted in a circular import, so I thought the easiest way to work
around that was to just create a new file.

pmb/helpers/exceptions.py

@@ -0,0 +1,5 @@
+class NonBugError(Exception):
+    """Exception which originates from a problem not caused by pmbootstrap's code. This
+    could for example be raised if there is an error in a package pmboostrap is
+    interacting with in some way."""
+    pass