add better error message for invalid initial guess

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (501e667) 87.92% compared to head (185fc14) 87.95%.

:exclamation: Current head 185fc14 differs from pull request most recent head 374bd9b. Consider uploading reports for the commit 374bd9b to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #147 +/- ## ========================================== + Coverage 87.92% 87.95% +0.02% ========================================== Files 13 13 Lines 2327 2332 +5 ========================================== + Hits 2046 2051 +5 Misses 281 281 ``` | [Flag](https://app.codecov.io/gh/baggepinnen/ControlSystemIdentification.jl/pull/147/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/baggepinnen/ControlSystemIdentification.jl/pull/147/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson) | `87.95% <100.00%> (+0.02%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files](https://app.codecov.io/gh/baggepinnen/ControlSystemIdentification.jl/pull/147?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson) | Coverage Δ | | |---|---|---| | [ext/ControlSystemIdentificationLSOptExt.jl](https://app.codecov.io/gh/baggepinnen/ControlSystemIdentification.jl/pull/147?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson#diff-ZXh0L0NvbnRyb2xTeXN0ZW1JZGVudGlmaWNhdGlvbkxTT3B0RXh0Lmps) | `98.50% <100.00%> (+0.12%)` | :arrow_up: |

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