Show only AGEPRO Read Input File Exception Messages in Dialog

[efletcherPIFSC]

Currently, openExistingAGEPROInputDataFileToolStripMenuItem_Click outputs the entire ex exception object to the dialog box.

https://github.com/PIFSCstockassessments/AGEPRO-GUI/blob/f27f4611311a624a0275b8a3385939b67e6caa06/FormAgepro.cs#L312-L316

To make it easier and consistent for the user to read the error, return only exception messages to the AGERPRO Input ReadFile error message dialog box.