RemoteOperations::exec_command returns error data in a corrected (expected) format (#154)

[dmitry-lipetsk]

RemoteOperations::exec_command does not translate an error message into UTF8 implicitly - function "normalize_error" was deleted.

It resolves a problem with "mutable" format of error data that is returned from this method.

Asserts were added.

This PR closes #154

[dmitry-lipetsk]

This PR fixes an original problem with tests but a problem with RemoteOperations::exec_command is more difficult.

We forces this method to raise ExecUtilException with binary message when encoding is not defined. It resolves a problem with tests. But we do not pay attention to the following things:

1. RemoteOperations::exec_command may throw another ExecUtilException with text (string) message:
   https://github.com/postgrespro/testgres/blob/5e9ecbcce3db6ade8f3bf59290e8d57d8dde19fa/testgres/operations/remote_ops.py#L82C19-L82C36

2. LocalOperations::exec_command always raises ExecUtilException with text messages:

testgres/testgres/operations/local_ops.py

Line 119 in 5e9ecbc

self._raise_exec_exception('Utility exited with non-zero code. Error `{}`', cmd, process.returncode, error or output)

Note that we may not decode error/output here.

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I think we have to stop appending an error data into an exception message (of ExecUtilException), because we do not know the correct codepage of this data. On Windows, for example, error and output may have cp1251 charset.

We can transfer this data as an additional property of ExecUtilException.

[dmitry-lipetsk]

It is an incorrect direction. Exception (ExecUtilException) must have a message with normal string.