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

src/umzug.ts

@@ -48,9 +48,14 @@ 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: ${cause.message}`
+    const msg = 'Non-error value thrown. See info for full props'
+    try {
+      return `${msg}: ${String(cause)}`
+    } catch {
+      //Null prototype object 'cause' would end up here
+      return msg
+    }
   }
 }
 

test/umzug.test.ts

@@ -685,7 +685,10 @@ describe('alternate migration inputs', () => {
           async up() {},
 
           async down() {
-            throw 'Some cryptic failure'
+            // This kind of error is thrown in MikroOrm migrations
+            const reallyCrypticFailure: any = Object.create(null)
+            reallyCrypticFailure.customText = 'Some cryptic failure'
+            throw reallyCrypticFailure
           },
         },
         {
@@ -705,7 +708,7 @@ describe('alternate migration inputs', () => {
     )
 
     await expect(umzug.down()).rejects.toThrowErrorMatchingInlineSnapshot(
-      `"Migration m1 (down) failed: Non-error value thrown. See info for full props: Some cryptic failure"`,
+      `"Migration m1 (down) failed: Non-error value thrown. See info for full props"`,
     )
   })