FLINK-38449: Makes error message for fractional values for MemorySize explicit

[navinvishy]

What is the purpose of the change

Currently for a fractional value for memory size, like 1.5g, the exception message is generated from failing to parse the string ".5g". Improving the exception handling for this case.

Verifying this change

Added a change in MemorySizeTest to make sure this is handled as expected.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[flinkbot]

CI report:

• f5d8581 Azure: SUCCESS

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[davidradl]

I suggest changing this to test the valid characters allowed - maybe with one or more regex's. I assume the unit should be 1 or more digits followed by a supported unit characters.

[navinvishy]

Yes the unit is 1 or more digits followed by unit. I have changed this to use a regex. Could you take another look? Thanks!