Exception in an import callback leaves the compiler in an inconsistent state, leading to `You shall not have another CompilerStack aside me` error on next compilation

Hi, I'm getting an internal exception error but I'm not too sure why. I'm running solc on an express server.

### Steps to getting this error:
1) Compile a contract that has invalid imports
2) Try to compile a contract that has valid imports
3) `solc.compile()` throws error

### Error i'm getting:
```
{
errors: [
{
component: 'general',
formattedMessage: 'Internal exception in StandardCompiler::compile: /solidity/libsolidity/interface/CompilerStack.cpp(107): Throw in function solidity::frontend::CompilerStack::CompilerStack(ReadCallback::Callback)\n' +
'Dynamic exception type: boost::wrapexcept\n' +
'std::exception::what: You shall not have another CompilerStack aside me.\n' +
'[solidity::util::tag_comment*] = You shall not have another CompilerStack aside me.\n',
message: 'Internal exception in StandardCompiler::compile: /solidity/libsolidity/interface/CompilerStack.cpp(107): Throw in function solidity::frontend::CompilerStack::CompilerStack(ReadCallback::Callback)\n' +
'Dynamic exception type: boost::wrapexcept\n' +
'std::exception::what: You shall not have another CompilerStack aside me.\n' +
'[solidity::util::tag_comment*] = You shall not have another CompilerStack aside me.\n',
severity: 'error',
type: 'InternalCompilerError'
}
]
}
```

Reproduce the sequence through solc.compile(): first compile a contract with invalid imports, then compile one with valid imports. Inspect the CompilerStack.cpp location reported in the error and the import read callback path. Done means a failed import compilation does not leave the next compilation throwing “You shall not have another CompilerStack aside me.”

Written by the indexing model from the issue text.