[ELF] Postpone "unable to move location counter backward" error (#66854)

The size of .ARM.exidx may shrink across `assignAddress` calls. It is
possible
that the initial iteration has a larger location counter, causing
`__code_size =
__code_end - .; osec : { . += __code_size; }` to report an error, while
the error would
have been suppressed for subsequent `assignAddress` iterations.

Other sections like .relr.dyn may change sizes across `assignAddress`
calls as
well. However, their initial size is zero, so it is difficiult to
trigger a
similar error.

Similar to https://reviews.llvm.org/D152170, postpone the error
reporting.
Fix #66836. While here, add more information to the error message.