Fix ERROR message in injection_point.c

Fix ERROR message in injection_point.c

This commit fixes an error message that failed to show the correct
function and library names when a function cannot be loaded.

While on it, adjust the call to load_external_function() so as this
ERROR can be reached, by making load_external_function() return NULL
rather than fail if a function cannot be found for a given injection
point.

Thinkos in d86d20f0ba79.