The GDDR5 SGRAM provides error detection on the data bus to improve system reliability. The device generates a checksum per byte lane for both READ and WRITE data and returns the checksum to the controller. Based on the checksum, the controller can decide if the data (or the returned CRC) was transmitted in error and retry the READ or WRITE command. The device itself does not perform any error correction.
EDC errors are they bad?
No necessarily, it means and Error happen inside your board, it has been Detected and Corrected (from these actions Error Detection and Correction). If you see errors it means that you card EDC circuit is working at least... sure 0 error is perfect, but some error is always better than undetected (and uncorrected) errors (that could lead to a crash).
Errors could occurs
when your GPU start to work (frequency change)
when the frequency is too high
when the memory voltage is too low (this might be unchangeable in Polaris card am I right? Confirmation needed...)
when the IMC (memory controller) voltage is too low
randomly (unlikely but can occurs)
when the timings are too tight
voltage is not clean enough (can be partially avoided with VRM oc, bios mods needed or third party software like VRMTool, PCB dependent)
Anyway, we are talking about errors in the communication between your IMC and your memory chips, the causes could be multiples but the solution is always the same, data resend: what has been corrupted must be resent again and again until it arrives intact.
To many errors are the reason why apparently high "stable" memory OC could perform worse that lower memory OC, the GPU isn't crashing but the errors could be so frequent that you card is waiting more time correcting errors than sending actual data.
The value is shown as "GPU Memory Errors", it counts errors only when the GPU is under load(3D).
Overclocking the video memory can generate invisible errors that most of the time are recovered by the gpu, but this process waste bandwidth and processing power. Is important to find a stable and error-free overclock for the memory. Because of this, you may get more FPS at 2100Mhz than 2150Mhz.