Remove SCM tag blocks rollback in some situations

When rolling back a preparation that is in a phase where no tag has been created yet, the remove-scm-tag phase blocks the rollback.

Problem is this reaction on the not successful removal:

https://github.com/apache/maven-release/blob/8aec7ff27bc02e02f334cb6ad18354153ab102ce/maven-release-manager/src/main/java/org/apache/maven/shared/release/phase/RemoveScmTagPhase.java#L125

The error should be changed to a warning.