Handle unable to create a blob store writer for the attachment

- Return AttachmentError when cannot  create a blob store writer for the attachment.

- Add warning messages to pin point where the issue is.

Source/CBLMultipartDocumentReader.m

@@ -255,8 +255,15 @@ - (BOOL) startedPart: (NSDictionary*)headers {
             // format generated by -[CBL_Pusher uploadMultipartRevision:]. CouchDB (as of 1.2) doesn't
             // output any headers at all on attachments so there's no compatibility issue yet.
             NSString* name = CBLUnquoteString([disposition substringFromIndex: 21]);
-            if (name)
-                _attachmentsByName[name] = _curAttachment;
+            if (name) {
+                if (_curAttachment)
+                    _attachmentsByName[name] = _curAttachment;
+                else {
+                    Warn(@"Cannot create a blob store writer for the attachement.");
+                    _status = kCBLStatusAttachmentError;
+                    return NO;
+                }
+            }
         }
     }
     return YES;

Source/CBL_BlobStore.m

@@ -314,10 +314,14 @@ - (instancetype) initWithStore: (CBL_BlobStore*)store {
         if (![[NSFileManager defaultManager] createFileAtPath: _tempPath
                                                      contents: nil
                                                    attributes: nil]) {
+            Warn(@"CBL_BlobStoreWriter: Unable to create a temp file at %@", _tempPath);
             return nil;
         }
         _out = [NSFileHandle fileHandleForWritingAtPath: _tempPath];
         if (!_out) {
+            BOOL exists = [[NSFileManager defaultManager] fileExistsAtPath: _tempPath];
+            Warn(@"CBL_BlobStoreWriter: Unable to get a file handle for the temp file at "
+                  "%@ (exists: %@)", _tempPath, (exists ? @"yes" : @"no"));
             return nil;
         }
         CBLSymmetricKey* encryptionKey = _store.encryptionKey;