[gdb/symtab] Use [in module %s] notation more consistently in dwarf errors

In gdb/dwarf2/read.c, I found a few strings "in module %s":
...
$ grep "in module %s" gdb/dwarf2/read.c | fgrep -v '['
		     "DIE at %s in module %s"),
      error (_("Dwarf Error: Dummy CU at %s referenced in module %s"),
    error (_("Dwarf Error: Cannot find DIE at %s referenced in module %s"),
	error (_("Dwarf Error: DIE at %s referenced in module %s "
      error (_("Dwarf Error: Dummy CU at %s referenced in module %s"),
    error (_("Dwarf Error: Cannot find DIE at %s referenced in module %s"),
...
that are not using the commonly used "[in module %s]" notation.  Fix these.

In one case, the string was also used in the middle rather than at the end of
the message, so fix that as well.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>