Improving ambiguous exception message for bad header names

[mrsaldana]

• I have considered the risk of behavior change or other zero migration impact (https://github.com/OpenLiberty/open-liberty/wiki/Behavior-Changes).
• If this PR fixes an Issue, the description includes "Fixes #FILLMEIN" or "Resolves #FILLMEIN" (verify release bug label if applicable: https://github.com/OpenLiberty/open-liberty/wiki/Open-Liberty-Conventions).
• If this PR resolves an external Known Issue (including APARS), the description includes "Fixes #FILLMEIN" or "Resolves #FILLMEIN".

Fixes #33209

[mrsaldana]

#build (view Open Liberty Personal Build - ❌ completed with errors/failures.)

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[LibbyBot]

Code analysis and actions

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[mrsaldana]

Build failures unrelated.