Improve error messages around rejected requests

A few lint changes to make linters happy, and an improvement to the error messages:

     Running `/home/spider/Projects/rust/target/debug/caramel-client-rs ca.modio.se Test_Certificate_Please_Reject`
2022-10-13 12:30:29,431 ERROR [caramel_client::network] Rejected CSR from server when fetching 'https://ca.modio.se/c0e090a7f8ff3cbc07b2fe476d743cf32ae6be340a5498b046ba2814666afef1':
 {"status":403,"title":"Forbidden","detail":null}
Server rejected CSR (Certificate Signing Request). Delete Test_Certificate_Please_Reject.csr and .key

And with the verbose flag:

     Running `/home/spider/Projects/rust/target/debug/caramel-client-rs -v ca.modio.se Test_Certificate_Please_Reject`
2022-10-13 12:12:06,784 DEBUG [caramel_client_rs] cmd_args: CmdArgs { server: "ca.modio.se", client_id: "Test_Certificate_Please_Reject", log_level: Debug }
2022-10-13 12:12:06,784 INFO [caramel_client_rs] Using caramel server: 'ca.modio.se' with client_id: 'Test_Certificate_Please_Reject'
2022-10-13 12:12:06,784 DEBUG [caramel_client_rs] Verifying private key file: "Test_Certificate_Please_Reject.key"
2022-10-13 12:12:06,784 DEBUG [caramel_client_rs] Verifying CA certificate file: "ca.modio.se.cacert"
2022-10-13 12:12:06,784 DEBUG [caramel_client::certs] Verify CA certificate
2022-10-13 12:12:06,785 DEBUG [caramel_client_rs] Verifying CSR file: "Test_Certificate_Please_Reject.csr"
2022-10-13 12:12:06,785 DEBUG [caramel_client::network] Probing: 'ca.modio.se' using default TLS settings
2022-10-13 12:12:06,865 DEBUG [caramel_client::network] Failed to connect with default TLS settings.
 [60] SSL peer certificate or SSH remote key was not OK (SSL certificate problem: unable to get local issuer certificate)
2022-10-13 12:12:06,865 DEBUG [caramel_client::network] Probing 'ca.modio.se' using '"ca.modio.se.cacert"' as CA certificate
2022-10-13 12:12:07,087 DEBUG [caramel_client::network] GET https://ca.modio.se/c0e090a7f8ff3cbc07b2fe476d743cf32ae6be340a5498b046ba2814666afef1, status=403
2022-10-13 12:12:07,087 ERROR [caramel_client::network] Rejected CSR from server when fetching 'https://ca.modio.se/c0e090a7f8ff3cbc07b2fe476d743cf32ae6be340a5498b046ba2814666afef1':
 {"status":403,"title":"Forbidden","detail":null}
Server rejected CSR (Certificate Signing Request). Delete Test_Certificate_Please_Reject.csr and .key

fixes: ModioAB/agile#2474