properly ignore permission denied error with mirror

[harshavardhana]

Community Contribution License

All community contributions in this pull request are licensed to the project maintainers
under the terms of the [Apache 2 license] (https://www.apache.org/licenses/LICENSE-2.0).
By creating this pull request I represent that I have the right to license the
contributions to the project maintainers under the Apache 2 license.

Description

properly ignore permission denied error with mirror

Motivation and Context

just try mc mirror /usr/share alias/bucket it will fail in the middle
without skipping on permission-denied errors.

How to test this PR?

just test with mc mirror /usr/share alias/bucket

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Optimization (provides speedup with no functional changes)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Fixes a regression (If yes, please add commit-id or PR # here)
• Unit tests added/updated
• Internal documentation updated
• Create a documentation update request here

[shtripat]

LGTM. One question.