Add better error logging to snapshot restart.

Produce a more verbose error message if failed to restart
from a snapshot.

include/exception.h

@@ -44,6 +44,7 @@
 		unsigned line;
 }
 + (id) alloc;
+- (void) log;
 @end
 
 
@@ -74,6 +75,7 @@
 
 - (id) init: (uint32_t)errcode_, ...;
 - (id) init: (uint32_t)errcode_ args: (va_list)ap;
+- (void) log;
 @end
 
 

mod/box/box.m

@@ -286,8 +286,11 @@ static int
 recover_row(struct tbuf *t)
 {
 	/* drop wal header */
-	if (tbuf_peek(t, sizeof(struct header_v11)) == NULL)
+	if (tbuf_peek(t, sizeof(struct header_v11)) == NULL) {
+		say_error("incorrect row header: expected %zd, got %zd bytes",
+			  sizeof(struct header_v11), (size_t) t->size);
 		return -1;
+	}
 
 	@try {
 		u16 tag = read_u16(t);
@@ -303,8 +306,10 @@ recover_row(struct tbuf *t)
 			say_error("unknown row tag: %i", (int)tag);
 			return -1;
 		}
-	}
-	@catch (id e) {
+	} @catch (tnt_Exception *e) {
+		[e log];
+		return -1;
+	} @catch (id e) {
 		return -1;
 	}
 

src/exception.m

@@ -49,6 +49,11 @@
 	}
 	return e;
 }
+
+- (void) log
+{
+	[self subclassResponsibility: _cmd];
+}
 @end
 
 @implementation SystemError
@@ -106,6 +111,12 @@
 	vsnprintf(errmsg, sizeof(errmsg), tnt_errcode_desc(errcode), ap);
 	return self;
 }
+
+- (void) log
+{
+	say_error("%s at %s:%d, %s", object_getClassName(self),
+		  file, line, errmsg);
+}
 @end
 
 
@@ -116,7 +127,7 @@
 	va_start(ap, errcode_);
 	[super init: errcode_ args: ap];
 
-	say_error("%s at %s:%d, %s", object_getClassName(self), file, line, errmsg);
+	[self log];
 
 	return self;
 }