cmd/asm: improve assembler error messages

cmd/asm: improve assembler error messages

Provide file/line numbers for errors when we have them.
Make the assembler error text closer to the equivalent errors from the compiler.

Abort further processing when we come across errors.
Fixes #53994

Change-Id: I4d6a037d6d713c1329923fce4c1189b5609f3660
Reviewed-on: https://go-review.googlesource.com/c/go/+/455276
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>