[opt](iceberg) opt the error msg when create db which already exists (a…

…pache#46334)

### What problem does this PR solve?

before:
```
mysql> create database cmytest;
ERROR 1105 (HY000): errCode = 2, detailMessage = Failed to create database: cmytest, error message is:null
```

after:
```
mysql> create database cmytest;
ERROR 1105 (HY000): errCode = 2, detailMessage = Failed to create database: cmytest: org.apache.doris.common.DdlException: errCode = 2, detailMessage = Can't create database 'cmytest'; database exists
```

fe/fe-core/src/main/java/org/apache/doris/datasource/iceberg/IcebergMetadataOps.java

@@ -29,6 +29,7 @@
 import org.apache.doris.common.ErrorReport;
 import org.apache.doris.common.UserException;
 import org.apache.doris.common.security.authentication.PreExecutionAuthenticator;
+import org.apache.doris.common.util.Util;
 import org.apache.doris.datasource.DorisTypeVisitor;
 import org.apache.doris.datasource.ExternalCatalog;
 import org.apache.doris.datasource.ExternalDatabase;
@@ -125,7 +126,7 @@ public void createDb(CreateDbStmt stmt) throws DdlException {
             });
         } catch (Exception e) {
             throw new DdlException("Failed to create database: "
-                    + stmt.getFullDbName() + ", error message is:" + e.getMessage(), e);
+                    + stmt.getFullDbName() + ": " + Util.getRootCauseMessage(e), e);
         }
     }