Alembic: avoid crashing when doing certain file operations.

Things like missing directories are now properly checked for, rather than
crashing Blender.

This also adds support for relative paths when opening an ABC file.

source/blender/editors/io/io_alembic.c

@@ -31,6 +31,9 @@
 #  include "BLI_winstuff.h"
 #endif
 
+#include <string.h>
+#include <errno.h>
+
 #include "MEM_guardedalloc.h"
 
 #include "DNA_mesh_types.h"
@@ -417,9 +420,20 @@ static int get_sequence_len(char *filename, int *ofs)
 	}
 
 	char path[FILE_MAX];
+	BLI_path_abs(filename, G.main->name);
 	BLI_split_dir_part(filename, path, FILE_MAX);
 
+	if (path[0] == '\0') {
+		/* The filename had no path, so just use the blend file path. */
+		BLI_split_dir_part(G.main->name, path, FILE_MAX);
+	}
+
 	DIR *dir = opendir(path);
+	if (dir == NULL) {
+		fprintf(stderr, "Error opening directory '%s': %s\n",
+		        path, errno ? strerror(errno) : "unknown error");
+		return -1;
+	}
 
 	const char *ext = ".abc";
 	const char *basename = BLI_path_basename(filename);
@@ -523,6 +537,10 @@ static int wm_alembic_import_exec(bContext *C, wmOperator *op)
 
 	if (is_sequence) {
 		sequence_len = get_sequence_len(filename, &offset);
+		if (sequence_len < 0) {
+			BKE_report(op->reports, RPT_ERROR, "Unable to determine ABC sequence length");
+			return OPERATOR_CANCELLED;
+		}
 	}
 
 	bool ok = ABC_import(C, filename, scale, is_sequence, set_frame_range,