Raise friendly error when SHA1 signing is unavailable

[luigi617]

Issue #, if available:

Description of changes:
Add friendly error message when SHA1 is not supported.

Command: .venv/bin/aws cloudfront sign --url http://example.com/hi --key-pair-id APKAEXAMPLE123 --private-key file:///tmp/pk.pem --date-less-than 2027-01-01
Error:

/home/ec2-user/aws-cli/.venv/lib64/python3.14/site-packages/urllib3/__init__.py:35: NotOpenSSLWarning: urllib3 v2 only supports OpenSSL 1.1.1+, currently the 'ssl' module is compiled with 'AWS-LC FIPS 5.2.0'. See: https://github.com/urllib3/urllib3/issues/3020
  warnings.warn(

aws: [ERROR]: Failed to sign the URL using the SHA1 hash algorithm: 7174 (AWS_ERROR_CAL_UNSUPPORTED_ALGORITHM): The specified algorithm is unsupported on this platform.. CloudFront signed URLs require RSA PKCS1 v1.5 signing with SHA1, which may be disabled by default on your platform. Enable SHA1 support in your system's crypto provider and try again. 

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