Improve error messages

[KristinaRiemer]

• Return error message if user uploads csv with more than one waterbody name

[KristinaRiemer]

Add error messages about user upload data in uu_processed?

[KristinaRiemer]

Display message on comparisons tab when no uu data has been uploaded yet saying just that

[KristinaRiemer]

Check for complete user upload data (required columns, etc.)

[KristinaRiemer]

Done in #42 and #44