Prevent launcher download corruption

[cbrunnkvist]

Fixes #6654 for both curl and wget cases, and provides feedback on why shasum is unavailable (forcing us to skip checksum verification).

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[cbrunnkvist]

The result of a { ... } block is decided by its final expression - and an if ... fi statement always succeeds (in contrast to e.g. Scala). Consequently, that && was always true.

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[cbrunnkvist]

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(in progress)

[lightbend-cla-validator]

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[lightbend-cla-validator]

Hi @cbrunnkvist,

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