the error will propagate up and be printed to the user at program
start time; at runtime, dlopen will just fail and leave a message for
dlerror.

previously, if mprotect failed, subsequent attempts to perform
relocations would crash the program. this was resulting in an
increasing number of false bug reports on grsec systems where rwx
permission is not possible in cases where users were wrongly
attempting to use non-PIC code in shared libraries. supporting that
usage is in theory possible, but the x86_64 toolchain does not even
support textrels, and the cost of keeping around the necessary
information to handle textrels without rwx permissions is
disproportionate to the benefit (which is essentially just supporting
broken library setups on grsec machines).

also, i unified the error-out code in map_library now that there are 3
places from which munmap might have to be called.