Inaccurate error message about encrypted PEM string

[lukpueh]

Description of issue or feature request:
Below error message in create_rsa_public_and_private_from_pem seems odd if the passed string is not encrypted and/or not a PEM, which is possible because PEMRSA_SCHEMA, which checks the input beforehand, just aliases
AnyString.

securesystemslib/securesystemslib/rsa_keys.py

Lines 686 to 687 in 208ea21

	raise securesystemslib.exceptions.CryptoError('RSA (public, private) tuple'
	' cannot be generated from the encrypted PEM string: ' + str(e))

Current behavior:
Error message is inaccurate for possible inputs

Expected behavior:
Fix error message or better sanitize inputs

[lukpueh]

The PEMRSA_SCHEMA == AnyString-issue is described in #183 (see first item)