Fix "can not convert object to primitive" nonsence error message

src/umzug.ts

@@ -48,9 +48,11 @@ export class MigrationError extends errorCause.ErrorWithCause<unknown> {
   }
 
   private static errorString(cause: unknown) {
-    return cause instanceof Error
-      ? `Original error: ${cause.message}`
-      : `Non-error value thrown. See info for full props: ${cause as string}`
+    if (cause instanceof Error) return `Original error: ${cause.message}`
+    const msg = 'Non-error value thrown. See info for full props'
+    if (typeof cause !== 'object' || (cause as {}).toString !== undefined) return `${msg}: ${String(cause)}`
+    //Null prototype object 'cause' would end up here
+    return msg
   }
 }
 

test/umzug.test.ts

@@ -709,6 +709,44 @@ describe('alternate migration inputs', () => {
     )
   })
 
+  test('null-prototype throwables are wrapped helpfully', async () => {
+    // Errors derived from null prototype can not be printed with toString(), make sure we do not fail printing them
+    // This kind of error is thrown in MikroOrm migrations
+    const umzug = new Umzug({
+      migrations: [
+        {
+          name: 'm1',
+          async up() {},
+
+          async down() {
+            const reallyCrypticFailure: any = Object.create(null)
+            reallyCrypticFailure.customText = 'Some cryptic failure'
+            throw reallyCrypticFailure
+          },
+        },
+        {
+          name: 'm2',
+
+          async up() {
+            const reallyCrypticFailure: any = Object.create(null)
+            reallyCrypticFailure.customText = 'Some cryptic failure'
+            throw reallyCrypticFailure
+          },
+          async down() {},
+        },
+      ],
+      logger: undefined,
+    })
+
+    await expect(umzug.up()).rejects.toThrowErrorMatchingInlineSnapshot(
+        `"Migration m2 (up) failed: Non-error value thrown. See info for full props"`,
+    )
+
+    await expect(umzug.down()).rejects.toThrowErrorMatchingInlineSnapshot(
+        `"Migration m1 (down) failed: Non-error value thrown. See info for full props"`,
+    )
+  })
+
   test('typescript migration files', async () => {
     require('ts-node/register')
     const syncer = fsSyncer(path.join(__dirname, 'generated/umzug/typescript'), {