Bad error when region has a space in it

[rcoh]

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Bug Report

When AWS_REGION or AWS_DEFAULT_REGION has a space in it, the SDK will refuse to construct a request in a very opaque way:

operation="ListTables" service="dynamodb" status="construction_failure" EndpointResolutionError(InvalidUri(InvalidUriChar))

Since this is actually a pretty common way to have accidentally broken environment variables, we should error out on this specific case.

To reproduce: export AWS_REGION=' us-east-1' and run any SDK command eg. dynamodb.list_tables()`