fix(config): also error when env var in config is prefixed or suffixed

[jonaskuske]

This PR fixes two issues:

1. The documentation claimed that the config would be used as-is if an env var is not found in the environment, but that's not how snmp_exporter behaves: if an env var is missing while --config.expand-environment-variables, it errors while loading the config.

I've updated the README to reflect that.

2. If an env var was part of a larger string, snmp_exporter would neither error nor leave the string as-is. Instead, os.Expand would silently replace the env vars found within the string with "".

I've updated the implementation to also error when the (missing) env var is found within a larger string. A common failure scenario is having a password like abcd$geh==1 in the config: with --config.expand-environment-variables, this password would silently be changed to abcd==1 (stripping the env var), so authentication against targets would fail despite having correct auths in the config.
Now, snmp_exporter errors and complains about the missing variable geh.