Add KeylimeTpmError support and change return type in tpm.rs

[leonjia0112]

Add support for tpm custom error type. This is aim on solving #40 for changing return type.
Implementation details:

1. Implement KeylimeTpmError enum has two contribution
   • error generate from TPM command execution
   • error generate from rust part of TPM interface
2. Implement From trait for Error:
   • serde_json::error::Error
   • std::num::ParseIntError
   • std::string::FromUtf8Error
   • std::io::Error
   • std::time::SystemTimeError
3. Upate return type of run() function in a tuple and a custom error
   Result<(String, String), TpmExecError>
4. Support return type of other function in tpm.rs with KeylimeTpmError, which has benefit in code efficient and early return efficient

[frozencemetery]

@mbestavros PTAL

[mbestavros]

@frozencemetery, @leonjia0112, things look good to me. Definitely makes the code much more readable, IMO -- a lot less boilerplate.

If I were to look for any improvement, it may help to make some of the more generic errors that KeylimeTpmError inherits a bit more verbose -- for example, it may be helpful for the FromUtf8 or serde_json errors to tell us where exactly it's going wrong, especially if they're being used in a lot of places.

Other than that, I've included a few minor typo fixes inline; nothing major.

[leonjia0112]

@mbestavros thanks for catching those typos. Those are handled.

But for the improvement, I wasn't very sure what you mean. Do you suggest putting additional information instead of just using the description function?