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:

• f5d85818abce1ad36e1e01a7f196e7022ad428f2 Azure: SUCCESS

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