ycb_astropy_pr13033

what a new run launched now would send

TimeSeries: misleading exception when required column check fails.
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### Description
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For a `TimeSeries` object that has additional required columns (in addition to `time`), when codes mistakenly try to remove a required column, the exception it produces is misleading.

### Expected behavior
<!-- What did you expect to happen. -->
An exception that informs the users required columns are missing.

### Actual behavior
The actual exception message is confusing:
`ValueError: TimeSeries object is invalid - expected 'time' as the first columns but found 'time'`

### Steps to Reproduce
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```python
from astropy.time import Time
from astropy.timeseries import TimeSeries

time=Time(np.arange(100000, 100003), format='jd')
ts = TimeSeries(time=time, data = {"flux": [99.9, 99.8, 99.7]})
ts._required_columns = ["time", "flux"]                                   
ts.remove_column("flux")

```

### System Details
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import sys; print("Python", sys.version)
import numpy; print("Numpy", numpy.__version__)
import erfa; print("pyerfa", erfa.__version__)
import astropy; print("astropy", astropy.__version__)
import scipy; print("Scipy", scipy.__version__)
import matplotlib; print("Matplotlib", matplotlib.__version__)
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```
Windows-10-10.0.22000-SP0
Python 3.9.10 | packaged by conda-forge | (main, Feb  1 2022, 21:21:54) [MSC v.1929 64 bit (AMD64)]
Numpy 1.22.3
pyerfa 2.0.0.1
astropy 5.0.3
Scipy 1.8.0
Matplotlib 3.5.1
```

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