Inconsistent error handling for data import

[eboileau]

Error handling for data import remains somewhat inconsistent. The interface DatasetPostRequest defines what information is transferred to the backend for data upload (management API). The source for upload is the full path to the temporary file_id. The error_summary (BedImportTooManyErrors) is defined by the TextFileReader.report_error, and reports the full path to the file_id, which is irrelevant for the user in general. This is also used in Comparison (dataset API), where the actual file name is available and used as a source, with useful information for the user. In handling other similar errors, the source is never reported e.g. in the case of BedImportEmptyFile, this is not shown in user_message, but in message. I don't know what are the security implications in all cases.

[eboileau]

For import, in particular for Comparison, could we handle separately cases where the file is not BED, e.g. some random text file?