On a quest to configure OpenOCD properly. First of all, the error I'm getting was described in an issue and seems like adding "reset_config none separate" fixes that. I got a new error with this:

openocd -f extra/fff.cfg -f extra/stm32f0-openocd.cfg -c "stm_flash `pwd`/main.bin" -c shutdown
Open On-Chip Debugger 0.9.0 (2015-09-02-10:42)
Licensed under GNU GPL v2
For bug reports, read
	http://openocd.org/doc/doxygen/bugs.html
Info : The selected transport took over low-level target control. The results might differ compared to plain JTAG/SWD
adapter speed: 1000 kHz
adapter_nsrst_delay: 100
none separate
srst_only separate srst_nogate srst_open_drain connect_deassert_srst
Info : Unable to match requested speed 1000 kHz, using 950 kHz
Info : Unable to match requested speed 1000 kHz, using 950 kHz
Info : clock speed 950 kHz
Info : STLINK v2 JTAG v17 API v2 SWIM v4 VID 0x0483 PID 0x3748
Info : using stlink api v2
Info : Target voltage: 3.504912
Info : stm32f0x.cpu: hardware has 4 breakpoints, 2 watchpoints
none separate
stm_erase
target state: halted
target halted due to debug-request, current mode: Handler HardFault
xPSR: 0xc1000003 pc: 0xfffffffe msp: 0xfffffffc
auto erase enabled
Info : device id = 0x10006444
Info : flash size = 16kbytes
Error: stm32x device protected
Error: failed erasing sectors 0 to 0

OK, so apparently the chip is write-protected. No problem, we can unlock it with "stm32x unlock 0". After running that, and resetting the chip, I finally got:

openocd -f extra/fff.cfg -f extra/stm32f0-openocd.cfg -c "stm_flash `pwd`/main.bin" -c shutdown
Open On-Chip Debugger 0.9.0 (2015-09-02-10:42)
Licensed under GNU GPL v2
For bug reports, read
	http://openocd.org/doc/doxygen/bugs.html
Info : The selected transport took over low-level target control. The results might differ compared to plain JTAG/SWD
adapter speed: 1000 kHz
adapter_nsrst_delay: 100
none separate
srst_only separate srst_nogate srst_open_drain connect_deassert_srst
Info : Unable to match requested speed 1000 kHz, using 950 kHz
Info : Unable to match requested speed 1000 kHz, using 950 kHz
Info : clock speed 950 kHz
Info : STLINK v2 JTAG v17 API v2 SWIM v4 VID 0x0483 PID 0x3748
Info : using stlink api v2
Info : Target voltage: 3.506483
Info : stm32f0x.cpu: hardware has 4 breakpoints, 2 watchpoints
none separate
stm_erase
target state: halted
target halted due to debug-request, current mode: Thread 
xPSR: 0xc1000000 pc: 0xfffffffe msp: 0xfffffffc
auto erase enabled
Info : device id = 0x10006444
Info : flash size = 16kbytes
target state: halted
target halted due to breakpoint, current mode: Thread 
xPSR: 0x61000000 pc: 0x2000003a msp: 0xfffffffc
wrote 1024 bytes from file /home/sheep/dev/3rdparty/stm32f0-discovery-basic-template/main.bin in 0.104813s (9.541 KiB/s)
target state: halted
target halted due to breakpoint, current mode: Thread 
xPSR: 0x61000000 pc: 0x2000002e msp: 0xfffffffc
verified 980 bytes in 0.030723s (31.150 KiB/s)
shutdown command invoked

Which seems to be a correct flashing.

All this thanks to advice from @jaromir.sukuba, great thanks!

Now let's see if I can get a blink example to work...