input: Write a function to generate a voltage signal with random noise using a Python script. The function should take the signal frequency and amplitude as inputs and return the generated signal. If the given frequency or amplitude is invalid, the function should return an error message.
  assert generate_voltage_signal(250, 1) != None
  assert isinstance(generate_voltage_signal(250, 1), np.ndarray)
  assert generate_voltage_signal(0, 1) == "Invalid frequency"
  assert generate_voltage_signal(250, -1) == "Invalid amplitude"
output: import numpy as np
  
  def generate_voltage_signal(frequency, amplitude):
      if frequency <= 0:
          return "Invalid frequency"
      if amplitude < 0:
          return "Invalid amplitude"
      
      time = np.linspace(0, 1, 1000)
      signal = amplitude * np.sin(2 * np.pi * frequency * time)
      noise = np.random.normal(0, 0.1, signal.shape)
      return signal + noise