Code creep: Avoid migration errors

..because of existing indices. I think this is unnecessary, e.g. in
development.

backend/src/db/migrations/20200207080200-fulltext_index_for_tags.js

@@ -15,12 +15,18 @@ export async function up(next) {
     await transaction.commit()
     next()
   } catch (error) {
+    const { message } = error
+    if (message.includes('There already exists an index')) {
+      // all fine
       // eslint-disable-next-line no-console
-    console.log(error)
+      console.log(message)
+      next()
+    } else {
       await transaction.rollback()
       // eslint-disable-next-line no-console
       console.log('rolled back')
       throw new Error(error)
+    }
   } finally {
     session.close()
   }

backend/src/db/migrations/20200213230248-add_unique_index_to_image_url.js

@@ -19,11 +19,18 @@ export async function up(next) {
     await transaction.commit()
     next()
   } catch (error) {
+    const { message } = error
+    if (message.includes('There already exists an index')) {
+      // all fine
       // eslint-disable-next-line no-console
-    console.log(error)
+      console.log(message)
+      next()
+    } else {
       await transaction.rollback()
       // eslint-disable-next-line no-console
       console.log('rolled back')
+      throw new Error(error)
+    }
   } finally {
     session.close()
   }