Print error message upon periodic surface failure in exo2nek

[aprilnovak]

When running exo2nek with periodic surfaces, the user is asked to enter the translation vector between the two surfaces. However, if the user makes a mistake entering this vector, exo2nek typically segfaults, like

 input translation vector (surface 1 -> surface 2)
0.0 0.0 22.91684

Program received signal SIGSEGV: Segmentation fault - invalid memory reference.

Backtrace for this error:
#0  0x7fdaf1942d21 in ???
#1  0x7fdaf1941ef5 in ???
#2  0x7fdaf162408f in ???
	at /build/glibc-SzIz7B/glibc-2.31/signal/../sysdeps/unix/sysv/linux/x86_64/sigaction.c:0
#3  0x55e7fa6dde2c in ???
#4  0x55e7fa6e2fed in ???
#5  0x55e7fa6da5be in ???
#6  0x7fdaf1605082 in __libc_start_main
	at ../csu/libc-start.c:308
#7  0x55e7fa6da5fd in ???
#8  0xffffffffffffffff in ???
Segmentation fault (core dumped)

It would help to provide a useful error message to the user before exiting gracefully. Depending on the nature of the seg fault, this error message could be something along the lines of Failed to find periodic surface at vector 0.0 0.0 22.91684. Did you enter the translation vector correctly?

Alternatively, it'd be even better to deduce the translation vector automatically given the two sideset IDs (just a distance calculation).