improve error handling and error messages

[robx]

The error messages are mostly poor across the board, on all levels. This issue is meant to collect some examples and possible improvements.

E.g.:

Bad request: Error in $: no solution provided

• web interface shouldn't expose HTTP codes, i.e., "Bad request: "
• aeson's "Error in $" is useless
• "no solution provided" is correct, but better to (a) explicitly say that the solution: key is missing, and (b) that we need it because we were asked to show the solution

In a Galaxies puzzle:

Bad request: Error in $: got 'X', expected 'o'

• got 'X', expected 'o' is pretty good, but saying explicitly that "galaxy centers are marked with 'o'" would be nicer

In a curve data puzzle (probably applies to everything with references).
After removing a cell:

Bad request: Error in $: expected a letter

• correct would be "expected a letter or '.'" (instead of a space)
• just deal with holes in the grid instead?

Reference to a missing letter:

Bad request: Error in $: mzero

• "missing reference: x"
• better: the grid references a clue 'x', which must be defined in the "clues" part

Inserting a bare : in coral clues:

Bad request: InvalidYaml (Just (YamlParseException {yamlProblem = "did not find expected node content", yamlContext = "while parsing a flow node", yamlProblemMark = YamlMark {yamlIndex = 105, yamlLine = 3, yamlColumn = 24}}))

• simply state that the YAML is broken
• provide a more userfriendly error message?