bpo-40899: Document exception raised when module cannot be imported

Doc/library/functions.rst

@@ -1910,7 +1910,8 @@ are always available.  They are listed here in alphabetical order.
       This is an advanced function that is not needed in everyday Python
       programming, unlike :func:`importlib.import_module`.
 
-   This function is invoked by the :keyword:`import` statement.  It can be
+   This function is invoked by the :keyword:`import` statement.
+   If the module cannot be imported, :exc:`ImportError` is raised. It can be
    replaced (by importing the :mod:`builtins` module and assigning to
    ``builtins.__import__``) in order to change semantics of the
    :keyword:`!import` statement, but doing so is **strongly** discouraged as it

Doc/library/importlib.rst

@@ -96,7 +96,7 @@ Functions
     specified in relative terms, then the *package* argument must be set to
     the name of the package which is to act as the anchor for resolving the
     package name (e.g. ``import_module('..mod', 'pkg.subpkg')`` will import
-    ``pkg.mod``).
+    ``pkg.mod``). If the module cannot be imported, :exc:`ImportError` is raised.
 
     The :func:`import_module` function acts as a simplifying wrapper around
     :func:`importlib.__import__`. This means all semantics of the function are