Move text of label to SNR_THRESHOLD_ERROR_MSG class-level variable
Add NO_CURVE_IN_ANY_FILTER_ERROR_MSG class-level variable
Add StarDetailsGUI.set_error_msg()
Show error message if all curve points are below the SNR threshold
Show error message if no light curve exists for the star
Disable 'Export' button if there are no light curves
Fix ValueError if we double click on stars without light curves

StarDetailsGUI.__init__() raises ValueError ("min() arg is an empty
sequence") when we double click on an astronomical object for which no
light curves were generated in any photometric filter, as we use min()
to determine the filter with which to begin. Fix this by taking into
account that there may be no curves for a star and, when this happens,
inform the user about it. Thanks so much to Scott Engle for reporting
this (#18).

This merges branch 'allow-stars-without-light-curves'