docs: release: Add note on panicking if flash device open fails

Adds a note that flash open failing will cause the bootloader to
panic and now prints a verbose error out when this happens

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
diff --git a/docs/release-notes.d/boot-open-failure.md b/docs/release-notes.d/boot-open-failure.md
new file mode 100644
index 0000000..25f80c2
--- /dev/null
+++ b/docs/release-notes.d/boot-open-failure.md
@@ -0,0 +1,2 @@
+- Add error when flash device fails to open.
+- Panic bootloader when flash device fails to open.