Print computed checksum on validation failure

[mbien]

simplifies the upgrade process a little bit by printing the computed checksum on verification failure.

Unix only.

example output:

Error: Failed to validate Maven distribution SHA-256, your Maven distribution might be compromised.
If you updated your Maven version, you need to update the specified distributionSha256Sum property.
expected: 1111111111111111111111111111111111111111111111111111111111111111
recieved: 0d7125e8c91097b36edb990ea5934e6c68b4440eef4ea96510a0f6815e7eeadb

for the following maven-wrapper.properties:

distributionType=only-script
distributionUrl=https://repo.maven.apache.org/maven2/org/apache/maven/apache-maven/3.9.11/apache-maven-3.9.11-bin.zip
distributionSha256Sum=1111111111111111111111111111111111111111111111111111111111111111

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