snapfs: more informative error messages for malformed snapshots
 [rsc] --rw-rw-r-- M 136171 glenda sys 4332 Mar  6 11:02 sys/src/cmd/snap/read.c
	/n/sourcesdump/2006/0306/plan9/sys/src/cmd/snap/read.c:136,142 - 
	/n/sourcesdump/2006/0307/plan9/sys/src/cmd/snap/read.c:136,142
	  	Page **pp;
	  	int i, npg;
	  	int t;
	- 	int len;
	+ 	int n, len;
	  	ulong pid;
	  	uvlong off;
	  	char buf[Pagesize];
	/n/sourcesdump/2006/0306/plan9/sys/src/cmd/snap/read.c:180,187 - 
	/n/sourcesdump/2006/0307/plan9/sys/src/cmd/snap/read.c:180,187
	  				fprint(2, "0x%.8llux same as %s pid %lud 0x%.8llux\n", s->offset+i*Pagesize, t=='m'?"mem":"text", pid, off);
	  			break;
	  		case 'r':
	- 			if(Bread(b, buf, len) != len)
	- 				panic("error reading segment xx");
	+ 			if((n=Bread(b, buf, len)) != len)
	+ 				sysfatal("short read of segment %d/%d at %llx: %r", n, len, Boffset(b));
	  			pp[i] = datapage(buf, len);
	  			if(debug)
	  				fprint(2, "0x%.8llux is raw data\n", s->offset+i*Pagesize);