Improve samtools view error reporting.

"[main_samview] truncated file" is a staggeringly unhelpful and
misleading catch-all for something went wrong.

Knowing precisely what isn't particularly trivial, but the next best
thing is reporting the system errno as it may at least point the
finger of blame somewhere sensible.

sam_view.c

@@ -679,6 +679,7 @@ int main_samview(int argc, char *argv[])
         if ((has_index_file && optind >= argc - 2) || (!has_index_file && optind >= argc - 1)) { // convert/print the entire file
             bam1_t *b = bam_init1();
             int r;
+            errno = 0;
             while ((r = sam_read1(in, header, b)) >= 0) { // read one alignment from `in'
                 if (!process_aln(header, b, &settings)) {
                     if (!is_count) { if (check_sam_write1(out, header, b, fn_out, &ret) < 0) break; }
@@ -688,7 +689,7 @@ int main_samview(int argc, char *argv[])
                 }
             }
             if (r < -1) {
-                fprintf(stderr, "[main_samview] truncated file.\n");
+                print_error_errno("view", "error reading file \"%s\"", fn_in);
                 ret = 1;
             }
             bam_destroy1(b);