bootloader: improve launch code

- Fix error not showing if payload is missing or can't be read
- Move errors to their callee function to save binary space
- Refactor various parameters and comments
- Reduce size on some errors
- Do not read HOS specific config in case of payload launch
- Remove unneeded code

bootloader/hos/fss.c

@@ -158,7 +158,7 @@ int parse_fss(launch_ctxt_t *ctxt, const char *path)
 	// Check if valid FSS0 and parse it.
 	if (fss_meta->magic == FSS0_MAGIC)
 	{
-		gfx_printf("Found FSS/PK3, Atmosphere %d.%d.%d-%08x\n"
+		gfx_printf("Atmosphere %d.%d.%d-%08x via FSS0/PKG3\n"
 			"Max HOS: %d.%d.%d\n"
 			"Unpacking..  ",
 			fss_meta->version >> 24, (fss_meta->version >> 16) & 0xFF, (fss_meta->version >> 8) & 0xFF, fss_meta->git_rev,