bootctl: refactor get_file_version() to use proper PE parsing

This irked me for a while. Let's not scan for strings stupidly, but
properly parse PE files to find the magic marker.

It's easy with our own PE APIs, hence we should do it.

This moves logging to the callers (previously, this was all mixed up).

src/bootctl/bootctl-install.c

@@ -462,13 +462,13 @@ static int version_check(int fd_from, const char *from, int fd_to, const char *t
         if (r == -ESRCH)
                 return log_notice_errno(r, "Source file \"%s\" does not carry version information!", from);
         if (r < 0)
-                return r;
+                return log_error_errno(r, "Failed to get file version of '%s': %m", from);
 
         r = get_file_version(fd_to, &b);
         if (r == -ESRCH)
                 return log_info_errno(r, "Skipping \"%s\", it's owned by another boot loader (no version info found).", to);
         if (r < 0)
-                return r;
+                return log_error_errno(r, "Failed to get file version of '%s': %m", to);
         if (compare_product(a, b) != 0)
                 return log_info_errno(SYNTHETIC_ERRNO(ESRCH),
                                       "Skipping \"%s\", it's owned by another boot loader.", to);
@@ -1806,6 +1806,8 @@ static int remove_boot_efi(InstallContext *c) {
                         return log_oom();
 
                 fd = xopenat_full(dirfd(d), de->d_name, O_RDONLY|O_CLOEXEC|O_NONBLOCK|O_NOCTTY|O_NOFOLLOW, XO_REGULAR, /* mode= */ MODE_INVALID);
+                if (fd == -ENOENT)
+                        continue;
                 if (fd < 0)
                         return log_error_errno(fd, "Failed to open '%s' for reading: %m", z);
 
@@ -1821,8 +1823,10 @@ static int remove_boot_efi(InstallContext *c) {
                 r = get_file_version(fd, &v);
                 if (r == -ESRCH)
                         continue;  /* No version information */
-                if (r < 0)
-                        return r;
+                if (r < 0) {
+                        log_warning_errno(r, "Failed to get file version of '%s', skipping: %m", de->d_name);
+                        continue;
+                }
                 if (!startswith(v, "systemd-boot "))
                         continue;